Foetal growth restriction is a pregnancy complication where an unborn baby does not grow as expected, measuring smaller than most babies at the same stage of pregnancy. This condition affects approximately one in every ten pregnancies and occurs when various factors prevent the baby from reaching its full growth potential, ranging from problems with the placenta to maternal health conditions.

Understanding Foetal Growth Restriction

Foetal growth restriction, also known as intrauterine growth restriction, occurs when a baby in the womb fails to grow at the rate expected for the number of weeks of pregnancy. Healthcare professionals define this condition as an estimated foetal weight below the 10th percentile for gestational age (the number of weeks of pregnancy). In simpler terms, this means the baby weighs less than nine out of ten babies conceived around the same time.^[1][2]

It’s important to understand that not every small baby has foetal growth restriction. Some babies are naturally small because their parents are small, and these babies are perfectly healthy. They are simply constitutionally small and fulfilling their genetic growth potential. The challenge for doctors is distinguishing between a naturally small baby and one whose growth is restricted due to an underlying problem. When growth restriction is present, it indicates that something is preventing the baby from achieving the size it should reach based on its genetic blueprint.^[3]

There are two main types of foetal growth restriction. Symmetrical growth restriction, also called primary restriction, means all parts of the baby’s body are similarly small in size. This type accounts for up to 30% of cases. Asymmetrical growth restriction, or secondary restriction, is more common, representing up to 80% of cases. In this type, the baby’s head and brain are the expected sizes, but the rest of the body, particularly the abdomen, is small.^[3]

How Common Is This Condition?

Foetal growth restriction affects approximately 10% of all pregnancies worldwide. This means that out of every ten pregnant women, about one will have a baby diagnosed with this condition. Despite this relatively high frequency, many babies diagnosed with growth restriction are born healthy and simply require additional monitoring during pregnancy and after birth.^[3][14]

The condition is usually detected after 20 weeks of pregnancy, when healthcare providers begin measuring fundal height at each prenatal visit. This simple measurement, taken from the top of the pubic bone to the top of the uterus, gives an initial indication of whether the baby is growing as expected. If the measurement is shorter than anticipated, the provider may suspect growth restriction and order additional tests to confirm.^[3]

What Causes Foetal Growth Restriction?

The causes of foetal growth restriction are diverse and can originate from the mother, the baby, or the placenta. Often, the condition develops because there is a problem with the placenta or umbilical cord, which are responsible for bringing nutrients, oxygen, and blood to the developing baby. When the placenta doesn’t attach properly to the uterine wall, or when blood flow through the umbilical cord is limited, the baby cannot receive adequate nourishment to grow properly.^[1][4]

Problems with the placenta represent the most common cause of foetal growth restriction. If the placenta is not functioning well, it cannot efficiently transfer nutrients and oxygen from the mother to the baby. This organ is essentially the baby’s lifeline during pregnancy, and any compromise in its function can directly impact foetal growth. The blood vessels in the placenta may be poorly developed, or the placenta may be too small to meet the baby’s needs.^[4]

Maternal Health Conditions

Several maternal health conditions increase the risk of foetal growth restriction. High blood pressure, whether it existed before pregnancy or develops during pregnancy, can restrict blood flow to the placenta and limit the baby’s growth. Women with diabetes, particularly when poorly controlled, face an increased risk of having a growth-restricted baby. Other maternal conditions that can contribute to growth restriction include heart disease, chronic kidney disease, and autoimmune conditions such as lupus.^[1][3]

Maternal anaemia (too few red blood cells) can also lead to growth restriction because red blood cells carry oxygen to the baby. Long-term lung or kidney conditions interfere with the mother’s overall health and her ability to support the baby’s growth. Being significantly underweight or having poor nutrition during pregnancy means the mother may not have adequate resources to support both her own body and the growing baby.^[1]

Lifestyle factors play a significant role in foetal growth restriction. Smoking cigarettes during pregnancy restricts blood vessels and reduces oxygen delivery to the baby. Alcohol consumption can interfere with normal foetal development and growth. The use of recreational drugs similarly affects the baby’s ability to receive proper nutrition and oxygen. These substances cross the placenta and directly impact the developing baby.^[1][3]

Foetal Factors

Sometimes the cause of growth restriction lies with the baby itself. Genetic conditions, such as an abnormal number of chromosomes (a condition known as aneuploidy), can affect how a baby grows. Down syndrome and other chromosomal abnormalities are associated with smaller foetal size. Birth defects, particularly those affecting major organs like the heart or digestive tract, can also result in growth restriction.^[1][4]

Certain infections during pregnancy can cause foetal growth restriction. These include cytomegalovirus, rubella (German measles), toxoplasmosis, and syphilis. When a pregnant woman contracts one of these infections, the infectious agent can cross the placenta and affect the baby’s development and growth. Multiple pregnancies, such as twins or triplets, commonly result in growth restriction because the babies must share the available nutrients and space. The odds of twins being born too small are as high as one in five, while for triplets, the odds increase to three in five.^[4][5]

Risk Factors for Developing Growth Restriction

Certain groups of women face a higher risk of having a baby with foetal growth restriction. Women who have previously had a baby with growth restriction are at increased risk in subsequent pregnancies. This suggests that some underlying maternal factors may persist across pregnancies, whether related to the placenta, blood flow, or other biological mechanisms.^[3]

Taking certain medications during pregnancy can increase the risk of growth restriction. Some anti-seizure medications, for example, are known to affect foetal growth. Women who require these medications should work closely with their healthcare providers to balance the need for treatment with potential risks to the baby. Exposure to certain harmful chemicals in the environment or workplace can also contribute to growth restriction.^[3][4]

Living at high altitudes has been associated with lower birth weight and an increased risk of growth restriction. At higher elevations, the amount of oxygen available in the air is reduced, which can affect the amount of oxygen reaching the baby through the placenta. Women living in mountainous regions or at significant elevations may need additional monitoring during pregnancy.^[3]

Symptoms and Signs

One of the challenging aspects of foetal growth restriction is that pregnant women typically don’t experience any symptoms themselves. The mother cannot feel whether her baby is growing properly just by paying attention to her body. This is why regular prenatal care is so important for detecting this condition. Healthcare providers rely on measurements and tests rather than maternal symptoms to identify growth restriction.^[1][6]

However, babies with growth restriction may show certain signs after birth. These newborns often have low birth weight, meaning they weigh less than expected for the number of weeks they spent in the womb. They may experience low blood sugar levels shortly after delivery, a condition called hypoglycaemia. This happens because small babies have limited energy stores and their bodies struggle to regulate blood sugar on their own.^[1]

Newborns affected by growth restriction may have difficulty maintaining their body temperature. They don’t have enough body fat to provide insulation and generate heat, so they get cold more easily than babies of normal size. Some growth-restricted babies have an unusually high level of red blood cells in their blood, a condition that developed in the womb as the baby’s body tried to compensate for reduced oxygen delivery. These babies may also have more trouble fighting off infections because their immune systems are less developed.^[1][4]

In more severe cases of growth restriction, babies may show physical signs that indicate poor growth during pregnancy. They may appear thin and pale, with loose, dry skin. The umbilical cord might be thinner and duller in appearance than normal. These physical characteristics reflect the chronic lack of adequate nutrition the baby experienced in the womb.^[21]

Prevention Strategies

Unfortunately, there are no proven evidence-based measures specifically for preventing foetal growth restriction. Nutritional treatments and dietary supplements have not been shown in research studies to prevent this condition. However, women can take steps to optimize their health before and during pregnancy, which may reduce some risk factors associated with growth restriction.^[14]

Eating a balanced, nutritious diet throughout pregnancy provides the building blocks the baby needs for growth. While diet alone cannot prevent growth restriction if other factors are present, poor nutrition can certainly contribute to the problem. Maintaining a healthy weight before pregnancy and gaining the appropriate amount of weight during pregnancy supports foetal development.^[4]

Avoiding smoking, alcohol, and recreational drugs is crucial for preventing growth restriction and many other pregnancy complications. These substances directly harm the developing baby and interfere with the placenta’s ability to function properly. Women who smoke should seek help to quit before becoming pregnant or as soon as they learn they are pregnant. Similarly, any alcohol consumption during pregnancy should be avoided.^[4]

For women with pre-existing health conditions such as diabetes, high blood pressure, or autoimmune diseases, following their treatment plan and keeping these conditions well-controlled can help reduce the risk of growth restriction. Regular prenatal care allows healthcare providers to monitor both the mother’s health and the baby’s growth closely. Early detection and management of pregnancy complications like high blood pressure can make a significant difference.^[4]

Low-dose aspirin is sometimes prescribed to women at moderate or high risk of developing preeclampsia, a serious pregnancy complication involving high blood pressure. While aspirin is not prescribed solely to prevent foetal growth restriction, its use for preeclampsia prevention may have the added benefit of reducing the likelihood of developing growth restriction. However, aspirin should only be taken during pregnancy under medical supervision and when specifically recommended by a healthcare provider.^[14]

How the Body’s Normal Functions Are Affected

Understanding what happens in the body when foetal growth restriction develops helps explain why this condition can have serious consequences. Normal foetal growth depends on a complex interplay between the mother’s body, the placenta, and the baby. When any part of this system doesn’t work properly, the baby’s growth suffers.^[2]

The placenta serves as the interface between mother and baby, allowing nutrients and oxygen to pass from the mother’s bloodstream into the baby’s circulation while removing waste products. In healthy pregnancies, the placenta grows and adapts to meet the baby’s increasing needs as pregnancy progresses. Blood vessels in the placenta normally expand and multiply to ensure adequate blood flow. When placental development is abnormal or blood flow is restricted, this transfer of nutrients and oxygen becomes inadequate.^[2]

In response to chronic oxygen deprivation, the baby’s body makes adaptations to protect the most vital organs, particularly the brain and heart. Blood flow is redistributed, sending more blood to these critical organs and less to other parts of the body. This explains why asymmetrical growth restriction occurs, with the head appearing normal-sized while the body, especially the abdomen, is small. The baby is essentially sacrificing growth in less critical areas to ensure the brain continues developing.^[3]

The baby’s metabolism also changes in response to inadequate nutrition. Energy stores in the form of fat and glycogen (stored sugar) become depleted. This is why growth-restricted babies often experience low blood sugar after birth—they simply don’t have the reserves that normally sized babies accumulate during pregnancy. The lack of adequate nutrition also affects the development of various organ systems, which can have lasting consequences even after birth.^[17]

Growth restriction that begins early in pregnancy, before 32 weeks of gestation, tends to be more severe and is more likely to be associated with genetic abnormalities, infections, or significant maternal health problems. When restriction begins later in pregnancy, it is more commonly due to placental insufficiency or maternal conditions like high blood pressure. The timing of when growth restriction begins and how severe it becomes influences both the immediate risks to the baby and the long-term outcomes.^[2][14]

The amniotic fluid that surrounds the baby can also be affected by growth restriction. When the placenta isn’t functioning well and the baby isn’t growing properly, the amount of amniotic fluid may decrease. This happens because the baby produces less urine, which is a major component of amniotic fluid in the second half of pregnancy. Low amniotic fluid, combined with growth restriction, signals that the baby is under significant stress.^[5]

The body’s response to foetal growth restriction continues after birth. Growth-restricted newborns face challenges that healthy-weight babies don’t typically encounter. Their small size and limited energy reserves make it difficult for them to maintain normal blood sugar levels and body temperature. They may struggle with feeding, whether breastfeeding or bottle-feeding, because they lack the strength and coordination that comes with more mature development. Their immune systems are less robust, making them more susceptible to infections.^[17]

Beyond the immediate newborn period, children who experienced growth restriction before birth may face long-term health challenges. They are at increased risk of abnormal growth patterns in childhood, meaning they may remain smaller than their peers or, conversely, may experience rapid catch-up growth that increases the risk of obesity later in life. Studies have found associations between foetal growth restriction and later development of metabolic syndrome, cardiovascular disease, high blood pressure, and diabetes in adulthood. Some children may experience neurodevelopmental challenges or learning difficulties.^[14][19]