Selective intrauterine growth restriction is a pregnancy complication in which one twin grows much less than the other. It happens most often in identical twins who share one placenta, and many pregnant people have no clear symptoms at first. A routine checkup can sometimes be the first step toward diagnosis, especially when early signs of selective intrauterine growth restriction are seen on ultrasound. Not everyone will have the same experience, but selective intrauterine growth restriction can raise the risk of poor growth, early delivery, and in severe cases loss of one or both babies. Treatment usually focuses on close ultrasound monitoring, managing complications, and sometimes planned early delivery or specialist fetal procedures depending on how far along the pregnancy is.
How many people live with Selective intrauterine growth restriction?
Because selective intrauterine growth restriction is a pregnancy complication, not a lifelong condition, it is usually estimated by counting affected twin pregnancies rather than people living with it: worldwide, about 1.6 million twin pregnancies happen each year (Pison et al., 2021), and since about 20% involve twins sharing one placenta and about 10% to 15% of those pregnancies are affected, this suggests roughly 32,000 to 48,000 affected pregnancies globally each year, not a direct count (ISUOG, 2016). In the United States, the twin birth rate was 31.2 per 1,000 live births in 2022, which works out to about 57,000 twin pregnancies (CDC/NCHS, 2024); using the same percentages suggests roughly 1,100 to 1,700 affected pregnancies a year, again an estimate rather than a direct count (CDC/NCHS, 2024; ISUOG, 2016).
Overview
Selective intrauterine growth restrictionSymptoms
Selective intrauterine growth restriction often causes no clear symptoms. It is usually found on ultrasound when one twin is much smaller than the other. In severe cases, you may notice less movement from one baby.
Outlook and prognosis
The outlook for selective intrauterine growth restriction depends mainly on how unequal the twins’ growth is, how early it starts, and blood flow in the smaller twin’s cord. With close ultrasound monitoring, many pregnancies reach delivery safely, though preterm birth is common.
Causes and risk factors
Selective intrauterine growth restriction usually stems from unequal placental sharing in identical twins who share one placenta. Abnormal blood-vessel connections can worsen it. Risks rise with monochorionic twin pregnancy, placental problems, and sometimes differences in cord insertion.
Genetic influences
Genetics usually play a limited role in selective intrauterine growth restriction. In most cases, selective intrauterine growth restriction is linked more to how the twins share the placenta than to inherited genetic variations, though genetic testing may be considered in some pregnancies.
Diagnosis
Selective intrauterine growth restriction is usually diagnosed by ultrasound showing one twin is much smaller than the other, often with repeated growth checks. Doppler blood flow studies and amniotic fluid measurements help confirm the diagnosis of selective intrauterine growth restriction.
Treatment and drugs
Selective intrauterine growth restriction is treated with close ultrasound and Doppler checks, careful timing of delivery, and monitoring of both twins. Some pregnancies need hospital care. In severe cases, fetal therapy may be discussed.
Symptoms
Selective intrauterine growth restriction often does not cause obvious symptoms for the pregnant person, even though one twin is growing more slowly than the other. The early symptoms of selective intrauterine growth restriction are often absent, which is why routine prenatal ultrasounds are so important. You might notice small changes at first, such as less movement from one baby, but many people feel completely well. Most warning signs are found on scans and fetal monitoring rather than through day-to-day symptoms.
Many pregnancies with selective intrauterine growth restriction do not cause pain, bleeding, or a noticeable change in how you feel. It is often found during a routine prenatal visit rather than because of symptoms at home.
Some people notice that one baby seems less active than expected, especially later in pregnancy. Reduced fetal movement can happen for different reasons, but it should be checked promptly.
Ultrasound may show that one twin is measuring smaller and gaining weight more slowly over time. In twin pregnancies, belly measurements alone often cannot show this clearly.
In selective intrauterine growth restriction, Doppler ultrasound may show reduced or abnormal blood flow in the umbilical cord of the smaller twin. This is a medical sign that helps the care team judge how stressed the baby may be.
How people usually first notice Selective intrauterine growth restriction?
Selective intrauterine growth restriction is usually first noticed during routine ultrasound scans in a twin pregnancy, most often when one baby is measuring much smaller than the other or growing more slowly over time. Doctors may also see differences in amniotic fluid, blood flow in the umbilical cord, or how well the placenta is supporting each twin, which helps explain how selective intrauterine growth restriction is first noticed rather than something a pregnant person can usually feel at home. For many families, the first signs of selective intrauterine growth restriction are not symptoms but unexpected scan findings that lead to closer monitoring and follow-up imaging.
Types of Selective intrauterine growth restriction
Selective intrauterine growth restriction can affect twins in different ways, and day-to-day concerns often center on how closely growth and blood flow need to be watched during pregnancy. In monochorionic twin pregnancies, doctors usually describe three main types of selective intrauterine growth restriction based on blood flow patterns in the smaller twin’s umbilical cord. These types of selective intrauterine growth restriction can differ in how stable the pregnancy seems and how likely it is that closer monitoring or earlier delivery will be needed. Not everyone will follow the same pattern, and the outlook can vary from one pregnancy to another.
Type I
Type I usually has normal forward blood flow in the smaller twin’s umbilical artery. This pattern is often the most stable, and many pregnancies can continue with careful monitoring before delivery.
Type II
Type II means there is persistently absent or reversed blood flow in the smaller twin’s umbilical artery. This type is generally more concerning because it can signal ongoing strain on the smaller twin and may lead to closer follow-up or earlier delivery.
Type III
Type III has an intermittent pattern, where umbilical artery blood flow switches back and forth between normal and absent or reversed. This can be harder to predict, so many living with selective intrauterine growth restriction in this group need very careful surveillance because the risk can change suddenly.
Can I slow down the onset of symptoms?
Not really; there is no good evidence that lifestyle changes can meaningfully delay the start of selective intrauterine growth restriction, where one baby in a twin pregnancy grows more slowly than the other, because it is usually caused by how the shared placenta develops and how blood flows through it. General healthy pregnancy steps like not smoking and keeping blood pressure and blood sugar well controlled are important for overall health, but they have not been shown to prevent this condition; the most important medical step is early and regular prenatal care with frequent ultrasound checks, especially in twins sharing one placenta. This close monitoring cannot guarantee a different outcome, but it gives doctors the best chance to act early and support both babies.
What genetics can tell you about symptoms
In selective intrauterine growth restriction, genetics usually does not determine the main symptoms or how severe they are; this condition is most often linked to how the placenta is shared between twins and how blood flows to each baby. Genetic differences or chromosome problems can sometimes be present in one twin, especially if the growth gap is very large or there are other unusual findings on ultrasound, so testing may help explain some cases. Overall, though, genetics is better at ruling out other causes than predicting exactly when problems will start or how the condition will behave.
Causes and risk factors
The biggest risk for selective intrauterine growth restriction is a twin pregnancy in which both babies share one placenta. This often happens because the placenta is not shared evenly, so one baby gets less blood flow and fewer nutrients. Problems with where the umbilical cord attaches, and unusual blood vessel connections between the twins, can also raise the chance of selective intrauterine growth restriction in twins. Some risks are modifiable (things you can change), others are non-modifiable (things you can’t). Smoking, poor nutrition, high blood pressure, and sometimes a genetic problem in one baby may worsen growth problems, but placental factors are usually the main cause.
What are the most common causes or risk factors?
The main risk factor is being pregnant with twins who share one placenta, especially identical twins, because one baby may end up with a smaller or less effective share of the placenta. A common cause is that the smaller baby’s umbilical cord attaches in a poor spot or the shared placenta has uneven blood flow, so that baby gets less oxygen and fewer nutrients. Less often, a problem with the baby’s development, including a genetic condition, or pregnancy issues that affect the placenta, such as high blood pressure or smoking, can also play a part.
Environmental and biological risk factors
Selective intrauterine growth restriction most often develops in twin pregnancies where both babies share one placenta. Doctors often group risks into internal (biological) and external (environmental). For this condition, the best-known risks are mainly biological and center on how the placenta and blood supply are divided between the twins.
Selective intrauterine growth restriction is seen mainly in monochorionic twins, which means the twins share one placenta. That shared setup makes uneven blood and nutrient delivery possible from early pregnancy.
One of the strongest risk factors is an uneven split of the placenta between the twins. If one baby receives a much smaller placental area, growth can slow because less oxygen and fewer nutrients reach that twin.
Risk rises when the smaller twin's umbilical cord attaches at the edge of the placenta or into the membranes instead of the stronger central area. This kind of attachment can make blood flow less steady and less efficient.
Blood vessel links between the twins inside the placenta can affect how blood moves back and forth. Certain patterns may leave one twin with poorer support or less ability to compensate for a smaller placental share.
Genetic risk factors
Selective intrauterine growth restriction is usually not something families pass down, and a clear inherited gene cause is uncommon. Doctors may still look into the genetic causes of selective intrauterine growth restriction when one twin is much smaller than the other, especially if scans show birth defects or other unusual findings. In some pregnancies, the smaller twin has a chromosome condition, a rare gene disorder, or a genetic change affecting the placenta. Risk is not destiny—it varies widely between individuals.
Chromosome changes: A change in chromosome number or structure in one twin can slow growth before birth. Doctors are more likely to suspect this when the smaller twin also has differences seen on ultrasound. Testing during pregnancy or after birth can sometimes confirm the cause.
Single-gene conditions: Rare disorders caused by a change in one gene can limit growth in the womb. These changes may be inherited or may appear for the first time in the pregnancy. They are more likely to be considered when slow growth comes with other physical differences on ultrasound.
Placental mosaicism: Sometimes the genetic change is mainly in placental cells rather than in the baby’s cells. This can affect how well nutrients and oxygen reach one twin and may contribute to selective intrauterine growth restriction. A placental test and a baby’s test do not always show the same result.
Inherited conditions: If a parent carries a genetic condition linked to poor fetal growth or birth defects, the chance of an inherited cause is higher. Clues can include repeated pregnancy losses, previous babies with growth problems, or a known family diagnosis. In some cases, genetic counseling can help sort out whether testing would be useful.
Lifestyle risk factors
Selective intrauterine growth restriction is not caused by a pregnant person's everyday habits, but maternal lifestyle can still influence placental function, fetal growth support, and the chance of complications. In practice, how lifestyle affects selective intrauterine growth restriction is mainly through factors that change oxygen delivery, nutrition, blood flow, and adherence to monitoring. The most useful focus is on habits that support the pregnancy and help clinicians detect deterioration early.
Smoking reduces oxygen delivery and can worsen placental blood flow, which may further limit growth in the smaller fetus. Stopping smoking during pregnancy can improve the oxygen environment and may lower the risk of additional fetal stress.
Alcohol does not cause selective intrauterine growth restriction itself, but it can add fetal growth and neurodevelopmental harm on top of an already vulnerable pregnancy. Avoiding alcohol removes a preventable source of stress for both fetuses.
Cocaine, methamphetamine, and other non-prescribed drugs can constrict blood vessels and reduce placental perfusion, which may worsen growth restriction or trigger acute complications. Avoiding recreational drugs and discussing all substances with the care team is especially important in this setting.
Poor calorie, protein, iron, and micronutrient intake can make it harder to meet the demands of a high-risk twin pregnancy. A well-supported eating pattern may help maternal health and optimize the nutrients available to the fetuses, even though it cannot correct the underlying placental imbalance.
Risk prevention
Selective intrauterine growth restriction can happen in some twin pregnancies, especially when twins share one placenta and blood flow is uneven between them. It usually cannot be fully prevented, because much of the risk comes from how the placenta develops early in pregnancy. Prevention is about lowering risk where possible and finding problems early, since early symptoms of selective intrauterine growth restriction are often not noticeable to the pregnant person. Prevention works best when combined with regular check-ups.
Booking prenatal care early gives your care team the best chance to identify a twin pregnancy and watch growth closely from the start. This is especially important when twins share a placenta, because selective intrauterine growth restriction is more likely in that setting.
Regular ultrasound scans can track each twin’s growth, amniotic fluid, and blood flow through the placenta and umbilical cord. These checks do not prevent every case, but they can catch selective intrauterine growth restriction sooner and guide safer timing of treatment or delivery.
Many living with a higher-risk twin pregnancy benefit from care with a maternal-fetal medicine specialist. Different people need different prevention strategies—there’s no single formula.
Not smoking and avoiding nicotine, cannabis, and recreational drugs can help protect placental blood flow and fetal growth. If stopping is hard, asking for support early in pregnancy can make a real difference.
Can you prevent Selective intrauterine growth restriction?
Selective intrauterine growth restriction usually cannot be fully prevented, because it often happens when twins sharing one placenta do not get an equal blood supply and one baby grows more slowly. Regular pregnancy checkups, not smoking, avoiding alcohol and drugs, and managing health problems such as high blood pressure may lower some risks and help catch it early. Even with excellent care, this can still happen, so early monitoring is often the most important step.
Transmission
Selective intrauterine growth restriction is not an infection, so it cannot be caught, passed from one person to another, or spread through blood, saliva, sex, or everyday contact. Instead, it develops during pregnancy, most often when twins share one placenta and one baby gets less blood flow and fewer nutrients than the other.
If you are wondering how selective intrauterine growth restriction is inherited, it usually is not inherited in a simple parent-to-child way and is not considered a contagious condition. For many families, this means the main issue is the shared placenta in that pregnancy, not infectiousness or something either parent did.
When to test your genes
For selective intrauterine growth restriction, genetic testing is not usually the first step because the problem is most often due to unequal placental sharing in identical twins, not an inherited condition. Testing may be discussed if ultrasound shows birth defects, very early or severe growth restriction, or other signs that suggest a chromosome or genetic disorder.
Diagnosis
Selective intrauterine growth restriction is usually picked up during pregnancy when one baby in a twin pregnancy is growing more slowly than the other. This often comes to light on routine prenatal ultrasound rather than from symptoms you can feel. Doctors usually begin with measurements of both babies, the amount of amniotic fluid, and how the placenta is shared, because these clues are central to the diagnosis of selective intrauterine growth restriction.
Ultrasound is the main test used to compare the babies’ size, estimated weight, and growth over time. A clear gap in growth, especially in twins sharing one placenta, raises concern that one baby is not getting the same support as the other.
Doctors confirm whether the twins share a placenta, because selective intrauterine growth restriction mainly affects monochorionic twins. This is done by looking at the membrane between the twins and other imaging features early in pregnancy.
Ultrasound also checks the fluid around each baby, since abnormal fluid levels can point to unequal placental function or a related twin complication. Fluid findings help doctors judge how severe the problem may be and whether another condition needs to be ruled out.
Doppler ultrasound measures blood flow in the umbilical cord and other fetal vessels. These flow patterns help show how well the smaller baby is being supplied and are a key part of how selective intrauterine growth restriction is diagnosed and monitored.
How is Selective intrauterine growth restriction diagnosed?
Selective intrauterine growth restriction is usually diagnosed during pregnancy with ultrasound scans that show one baby, most often in a twin pregnancy where the babies share one placenta, is much smaller than the other and smaller than expected for that point in pregnancy. Doctors also use a special ultrasound to check blood flow in the umbilical cord and placenta, and they usually repeat scans over time to track growth and the amount of fluid around each baby. Genetic testing is not usually what makes the diagnosis, but it may sometimes be offered if the growth pattern seems unusual or there are other signs that suggest a different cause.
Stages of Selective intrauterine growth restriction
Selective intrauterine growth restriction does not have defined progression stages. That is because it is a pregnancy complication that is identified by how one baby grows compared with the other, rather than a condition that moves through set phases over time. Doctors usually start with ultrasound scans to compare each baby’s size, growth pattern, amniotic fluid, and blood flow in the umbilical cord and other vessels. Many people feel reassured knowing what their tests can—and can’t—show, and these repeated scans are the main part of how selective intrauterine growth restriction is diagnosed and monitored during pregnancy.
Outlook and prognosis
The outlook for selective intrauterine growth restriction depends largely on how early it is detected, the severity of growth discordance, the type of placental sharing in twins, and whether complications such as abnormal Doppler blood flow or twin-to-twin transfusion are present. Mild cases monitored closely can result in survival of both babies with good outcomes, especially when growth remains stable and delivery is timed appropriately. More severe cases carry higher risks of preterm birth, fetal distress, neurologic injury, and loss of one or both fetuses, with prognosis generally worse in monochorionic twins because of shared placental circulation. Mortality is variable but is highest when there is absent or reversed end-diastolic flow, very early gestational onset, or sudden deterioration of the smaller twin. Even when both infants survive, the growth-restricted twin may face increased risks of neonatal intensive care admission, breathing problems, feeding difficulties, and later neurodevelopmental or metabolic complications. Care in a specialized fetal medicine center improves surveillance, guides decisions about intervention or delivery, and can improve the outlook for selective intrauterine growth restriction.
Long-term effects
Selective intrauterine growth restriction can have very different long-term effects depending on how early it starts, how severe it is, and whether the babies share a placenta. Many children who were the smaller twin do well over time, but some have lasting effects linked to poor growth before birth, early delivery, or both. Understanding long-term patterns can guide treatment choices. When people ask about the long-term effects of selective intrauterine growth restriction, doctors usually focus on growth, brain development, movement, and complications related to prematurity.
The smaller twin may remain shorter and lighter through infancy or childhood, especially if growth restriction was severe. Some catch up well after birth, while others stay smaller than their co-twin or than expected for family height.
Some children have delays in speech, language, problem-solving, or social development, particularly after very early or severe growth restriction. These delays can be mild and improve with time, but some need longer-term support.
Poor growth before birth and reduced oxygen supply can affect how the brain controls movement. In more severe cases, this can lead to muscle stiffness, coordination problems, or cerebral palsy, especially if there was also extreme prematurity.
School-age children may be more likely to have attention, memory, or learning difficulties than children from uncomplicated pregnancies. For many, symptoms are subtle at first and become clearer when school demands increase.
How is it to live with Selective intrauterine growth restriction?
Living with selective intrauterine growth restriction often means living with uncertainty during a twin pregnancy, because one baby is growing more slowly than the other and the pregnancy usually needs closer watching than expected. For many, this can mean more ultrasounds, more appointments, changes in work or daily routines, and the emotional strain of balancing hope with worry while doctors monitor both babies carefully. Partners, family, and other children may feel that stress too, as plans can shift quickly and extra practical and emotional support often becomes part of everyday life.
Treatment and drugs
Selective intrauterine growth restriction can turn a twin pregnancy into a series of extra scans and hard decisions, because one baby is growing much more slowly than the other and both babies may need closer watching. Treatment usually centers on careful monitoring rather than a single medicine, with frequent ultrasound checks of growth, blood flow, and each baby’s well-being so the team can decide when it is safest to continue the pregnancy and when early delivery may be needed. Treatment often looks different for each person, especially because selective intrauterine growth restriction can range from mild and stable to severe and fast-changing. In more serious cases, especially when twins share one placenta, care may include treatment at a specialist fetal medicine center and, in selected pregnancies, procedures to protect one or both babies by changing the blood vessel connections in the placenta or, rarely, stopping blood flow to one twin. Corticosteroid shots may be given before an early birth to help the babies’ lungs mature, and your doctor may also talk with you about the early symptoms of selective intrauterine growth restriction and the warning signs that need urgent review.
Can you cure Selective intrauterine growth restriction?
No, there is not a true cure for selective intrauterine growth restriction, which means one twin is growing much more slowly than the other during pregnancy. Care focuses on close monitoring, checking blood flow and growth, and deciding the safest time for delivery. In some pregnancies, specialists may also use procedures to lower the risk of serious problems, but these treatments manage the condition rather than cure it.
Non-drug treatment
Selective intrauterine growth restriction can mean one baby is growing much more slowly than the other, most often in a twin pregnancy, and day-to-day care usually centers on watching both babies closely and choosing the safest time for birth. Managing this condition often involves frequent visits with a maternal-fetal medicine specialist, because growth patterns and blood flow can change over days to weeks. The right treatment for selective intrauterine growth restriction in twins depends on how far along the pregnancy is, how different the babies’ growth is, and whether tests suggest one baby is under strain.
Care is usually led by a high-risk pregnancy team with regular checkups and repeat scans. This helps track growth, amniotic fluid, and each baby’s overall well-being so changes are picked up early.
Repeated ultrasound exams are a main part of care for selective intrauterine growth restriction. These scans compare the babies’ growth over time and can show whether the smaller baby is falling further behind.
Doppler ultrasound looks at blood flow in the umbilical cord and other vessels. It can show whether the placenta is struggling to support the smaller baby, which helps guide timing of delivery or other procedures.
Fetal heart rate monitoring may be used later in pregnancy to look for signs that a baby is not tolerating the pregnancy well. For many, treatment begins not at the pharmacy, but with this kind of close surveillance and timely decisions.
Your genes may have an impact on how drugs work for you
For people living with selective intrauterine growth restriction, medicines usually do not fix the growth problem itself, so care is more often close monitoring and, if needed, treatment around an early delivery. In general, inherited differences can change how some medicines are processed or how strongly they work, which may matter for drugs sometimes used during pregnancy or before a premature birth, but this is not a main part of care for this condition. Because treatment depends more on the babies’ growth, blood flow, and timing of delivery than on genes that affect drug response, doctors usually focus on ultrasound checks and pregnancy planning rather than genetic drug testing.
Pharmacological treatments
In selective intrauterine growth restriction, there is no medicine that can make the smaller fetus grow normally once the problem is established. Treatment usually focuses on timing delivery safely and, when early birth may happen, using medicines that protect the babies rather than treating the root cause. Medication can feel like a big step, but these drugs are most often used to lower risks around preterm birth and urgent delivery.
Betamethasone or dexamethasone may be given if doctors think preterm delivery is likely. These medicines help the babies’ lungs mature and can also lower the risk of some complications of very early birth. They are often part of care when selective intrauterine growth restriction raises concern for early delivery.
Magnesium sulfate may be used when very preterm birth is expected, usually to help protect the babies’ brains. It does not treat the growth problem itself, but it can reduce the risk of certain neurologic complications in very premature infants.
Medicines such as nifedipine, atosiban, or indomethacin may sometimes be used to briefly slow contractions if labor starts too soon. This short delay can give time for steroid treatment, transfer to a specialist center, or other urgent planning, but these drugs do not reverse the early symptoms of selective intrauterine growth restriction.
Genetic influences
When one baby in a twin pregnancy is much smaller than the other, many families worry about whether something genetic is to blame. In selective intrauterine growth restriction, genetics is only one piece of the puzzle, but genes in the baby or the placenta can sometimes affect growth. Most often, though, selective intrauterine growth restriction is linked more strongly to how the shared placenta developed and how blood flow is divided between the babies than to an inherited condition passed through the family. Doctors may think more about a genetic cause if the smaller baby also has birth defects, unusual ultrasound findings, or a very different growth pattern, and in those cases testing during pregnancy or after birth may be discussed. Changes in chromosomes or in genes that guide growth can occasionally be found, but many pregnancies with selective intrauterine growth restriction have no clear inherited genetic explanation. This is why the genetic part is usually assessed alongside the placenta, blood flow studies, and the baby’s overall development rather than on its own.
Is Selective intrauterine growth restriction genetic?
Genetics usually play a limited role in selective intrauterine growth restriction. In most cases, selective intrauterine growth restriction is linked more to how the twins share the placenta than to inherited genetic variations, though genetic testing may be considered in some pregnancies.
Having a genetic risk is not necessarily a diagnosis
Genetic variants, or changes in genes, are a usual part of human biology and are common across the general population. Most of these variants do not have a direct effect on health and are simply part of normal human diversity. For many genes, having a variant that is linked to a condition points to a possible association rather than a definite diagnosis, especially when several genes and environmental factors are involved. In some situations, though, a single well-understood variant in a specific gene can be enough to confirm a diagnosis.
Pharmacogenetics—how genetics influence drug effects
For many families, treatment for selective intrauterine growth restriction is less about finding the right drug and more about frequent ultrasound checks, blood-flow monitoring, and choosing the safest time for delivery. There is no medicine that can reliably reverse selective intrauterine growth restriction once it has developed. If the smaller twin's growth pattern or ultrasound findings suggest a chromosome change or another genetic condition, genetic testing may help explain the cause and guide counseling, but it usually does not change which medicines are used. Genetics is only one factor, and in selective intrauterine growth restriction the placental blood-flow pattern, how far along the pregnancy is, and the condition of both twins usually matter more for treatment decisions. Medicines that may be given around an early delivery, such as steroid shots to help the babies' lungs mature and magnesium sulfate to help protect the brain, are chosen mainly because of timing and risk, not because of a drug-response gene test. So when people ask about the role of genetics in treatment for selective intrauterine growth restriction in twins, the main value is often in understanding the cause and planning care, rather than matching the pregnancy to a specific medication.
Interactions with other conditions
Selective intrauterine growth restriction can become more complicated when other pregnancy problems are present, especially high blood pressure, preeclampsia, anemia, or problems with blood flow through the placenta. In day-to-day care, this can mean one twin is already growing more slowly, and then another condition further limits oxygen or nutrients, making close monitoring even more important. Doctors call it a “comorbidity” when two conditions occur together. Selective intrauterine growth restriction may also overlap with twin-to-twin transfusion syndrome in pregnancies where twins share one placenta, and that combination can raise the risk of distress before birth, preterm delivery, and health problems after delivery. Many living with selective intrauterine growth restriction are watched carefully for signs that the smaller twin is under strain and that the larger twin is also being affected, since complications in one baby can sometimes change the plan for both. Interactions can look very different from person to person, so care usually depends on how far along the pregnancy is, how the babies are growing, and whether there are early symptoms of selective intrauterine growth restriction getting worse on ultrasound or fetal monitoring.
How genes can cause diseases
Humans have more than 20,000 genes, each carrying out one or a few specific functions in the body. One gene instructs the body to digest lactose from milk, another tells the body how to build strong bones and another prevents the body's cells from beginning to multiply uncontrollably and developing into cancer. As all of these genes combined are the building instructions for our body, a defect in one of these genes can have severe health consequences.
Through decades of genetic research, we know the genetic code of any healthy/functional human gene. We have also identified that in certain positions on a gene, some individuals may have a different genetic letter from the one you have. We call these hotspots "Genetic Variations" or "Variants" in short. In many cases, studies have been able to show that having the genetic letter "G" in the position makes you healthy, but having the letter "A" in the same position disrupts the gene function and causes a disease. Genopedia allows you to view these variants in genes and summarizes all that we know from scientific research, which genetic letters (Genotype) have good or bad consequences on your health or on your traits.
What other conditions can cause similar symptoms to Selective intrauterine growth restriction?
In a twin pregnancy, similar concerns can happen if one baby is simply naturally smaller, if the placenta is not sharing blood and nutrients evenly, or if there is twin-to-twin transfusion syndrome, where blood flow between the twins becomes unbalanced. Doctors may also look for birth defects, genetic conditions, or infections during pregnancy, because these can also cause one baby to grow more slowly than expected.
History
For parents expecting twins, one of the hardest moments can be hearing that one baby is growing well while the other is falling behind. What is now called selective intrauterine growth restriction, or selective fetal growth restriction, was not always easy to recognize clearly because older doctors had no way to watch babies grow in the womb over time.
Earlier medical reports described twin pregnancies where one baby was much smaller than the other at birth, but the reason often remained uncertain. Before routine ultrasound, doctors usually had to piece the story together after delivery, using birth weights, the appearance of the placenta, and whether the babies shared it. This meant the early symptoms of selective intrauterine growth restriction during pregnancy were often missed, especially when the pregnant person felt otherwise well.
In recent decades, awareness has grown as ultrasound became a regular part of prenatal care. Doctors could then follow each twin’s growth more closely, measure blood flow, and see that this condition most often matters in twins who share a placenta, where the blood supply may not be evenly divided. That changed selective intrauterine growth restriction from something recognized mainly after birth into a condition that can often be spotted and monitored during pregnancy.
As understanding improved, the medical approach became more precise. Specialists learned that not all cases behave the same way: some remain fairly stable with close follow-up, while others carry a higher risk and need more frequent scans or earlier delivery. Today, the history of selective intrauterine growth restriction is closely tied to advances in prenatal imaging and fetal medicine, which have made it possible to detect problems earlier and support safer decisions for both babies and the pregnant parent.