What is Fetal Growth Restriction?
Fetal growth restriction (FGR) is a condition that affects 3% to 7% of all pregnancies. It’s a situation where a baby in the womb doesn’t grow as expected based on its genetic potential. To diagnose FGR, a process called Ultrasonography is used to estimate the weight of the fetus. If the weight is less than the 10th percentile for the specific gestational age (time the baby has spent in the womb), then the baby might have FGR. Some babies are naturally small, due to their genetic makeup, and are not growth-restricted. These babies are said to be small for their gestational age.
FGR can result from several conditions such as inborn disorders in the mother-baby-placenta unit, lack of enough nutrition for the fetus, or lack of space within the womb restricting the baby’s growth. FGR can lead to significant short-term and long-term health complications for the baby and can negatively affect their quality of life.
FGR is classified into different categories based on the severity determined by the estimated fetal weight. If the weight is between the 3rd and 9th percentile, it’s considered moderate FGR. If it’s less than the 3rd percentile, it’s deemed severe FGR.
FGR can also be categorized as symmetrical or asymmetrical, based on other measurements of the baby like the size of the head, abdomen, femur (thigh bone), and biparietal diameter (a measurement of the baby’s head). In symmetrical FGR, all growth measurements are relatively reduced. But in asymmetrical FGR, the size of the baby’s abdomen is less than the 10th percentile while other measurements remain normal.
Symmetrical FGR accounts for 20% to 30% of all FGR cases. It often occurs due to poor functioning of the placenta, the organ that supplies nutrients to the growing baby. It can also be caused by harmful conditions early in the pregnancy such as smoking, drug use, chronic high blood pressure, anemia, and long-term diabetes. Chromosome irregularities and certain infections are also major causes.
Asymmetrical FGR makes up 70% to 80% of all FGR cases. This type happens later in pregnancy, and while the baby’s head circumference remains normal, the abdomen’s size decreases. One common cause is a condition called preeclampsia, which is high blood pressure occurring during pregnancy, typically after the 20th week. This can lead to changes in the blood vessels in the placenta, reducing blood flow to the baby and compromising growth. Despite these issues, the baby’s brain often continues to grow normally due to a preferential blood supply.
What Causes Fetal Growth Restriction?
Fetal growth restriction (FGR) can be caused by issues related to the fetus, the placenta, or the mother. However, it’s often a mix of all three.
For the fetus, 5% to 20% of FGR cases can be due to genetic problems. These might be caused by abnormal numbers of chromosomes, misplaced genetic material, single-gene changes, and other rare conditions. If your baby isn’t growing as expected before 20 weeks of pregnancy, it might be due to an incorrect number of chromosomes. Infections can also cause FGR, accounting for 5% to 10% of cases. These are most often due to the cytomegalovirus or toxoplasmosis, but can also be caused by chickenpox, malaria, syphilis, or herpes. Sometimes, babies with non-genetic birth defects or certain syndromes can also have restricted growth.
Factors related to the mother can also cause FGR. This includes conditions like chronic high blood pressure, diabetes, autoimmune diseases, severe heart or kidney disease, severe anemia and malnutrition, sickle cell disease, and substance abuse. Certain types of medication for cancer or radiation exposure, chronic bleeding during pregnancy, being underweight before pregnancy or not gaining enough weight, being very young or old, short time between pregnancies, living at high altitude, having multiple pregnancies, abnormalities of the womb, or having fertility treatments can all affect the growth of the baby. A mother’s nutritional status can account for almost 10% of the variance in a baby’s weight. Interestingly, mothers who themselves had restricted growth are twice as likely to have babies with FGR.
Lastly, placental or umbilical cord issues can lead to FGR. Chromosomal differences in the placenta, with a normal baby, are found in 10% of unexplained FGR cases and 33% of those with placental damage and blood vessel issues in the womb. Other placental or umbilical cord abnormalities can also cause FGR. Diseases in mother can affect the growth of the baby by affecting the function of the placenta.
Risk Factors and Frequency for Fetal Growth Restriction
Fetal growth restriction (FGR), a condition where a baby doesn’t grow to normal weight while inside the womb, is found in around 3% to 7% of pregnancies. The number changes based on the specific group of people being studied, how old the unborn baby is, and whether babies smaller than the average for their gestational age were included in the study. This condition is 6 times more common in underdeveloped and developing countries compared to developed ones.
Approximately 20% of infants in low-income countries are smaller than they should be for their age at birth, and one in four of them may not survive. Most of the affected infants, about 75%, are found in Asia.
Women who have had preeclampsia and previously gave birth to a baby with FGR have a 20% chance of the same thing happening in their next pregnancy. FGR is quite puzzling, as there is no identifiable cause in about 40% of cases.
- In the remaining 60% of cases where a cause can be found:
- 1/3 are due to genetic anomalies.
- The rest are due to environmental factors.
Signs and Symptoms of Fetal Growth Restriction
A mother’s past health and habits can increase the chance of Fetal Growth Restriction (FGR). These include having a previous baby with FGR or preeclampsia, smoking or drug use, being pregnant with more than one baby, or having a history of chronic illnesses. Extreme ages of the mother can also be a factor. The woman’s fundal height, which is the distance from her pubic bone to the top of her uterus, might be smaller than normal.
The baby with FGR is typically in the lowest 10% for weight and might look thin with less muscle mass and fat. Depending on what’s causing the FGR, the baby’s head might look big or small relative to the rest of its body. The baby’s face might look thin, and its belly button cord could be shriveled. As a result of FGR, the baby’s skull might have wide separations in the bone and larger soft spots. The Ponderal index – a measure of the baby’s weight relative to its length – can show how severe the growth restriction is. If it’s below the 10th percentile, this suggests the baby is malnourished.
Depending on the reason for the FGR, the baby might show specific physical features at birth:
- Enlarged liver, hearing loss, eye inflammation, and blueberry muffin spots with a CMV infection.
- Low-set ears, split in the top of the mouth, clenched fist with overlapping fingers, and abnormal feet shape in trisomy 18.
- Scalp defect, close-set eyes, coloboma, undersized lower jaw, and belly button hernia in trisomy 13.
Other physical features of FGR might be linked to the root cause and may show up depending on the involved syndromes.
Testing for Fetal Growth Restriction
According to the American College of Obstetricians and Gynecology (ACOG), it’s important to measure the size of a pregnant woman’s belly (fundal height) during each prenatal doctor’s visit. If the belly size is more than 3 cm smaller or larger than what’s expected for the current week of pregnancy, an ultrasound test should be done. The ultrasound test can also see if there are any physical problems with the baby.
In addition, the ultrasound test can confirm the baby’s age in the womb. This is crucial for telling the difference between a baby that’s smaller than expected because it’s younger than thought (a miss-dated pregnancy), and a baby that’s not growing as it should (fetal growth restriction, or FGR).
The guidelines also highlight the importance of early identification of high-risk pregnancies. For example, a mother who had a baby with FGR in the past, uses harmful substances (like tobacco or alcohol), is older, has high blood pressure during pregnancy (preeclampsia), or had a previous pregnancy complicated by preeclampsia. If any risk factors are identified, regular ultrasounds should be done. If FGR is detected, tests should be done to check the amount of fluid around the baby (amniotic fluid volume estimations) and the blood flow in the baby’s umbilical artery (umbilical arterial Doppler blood flow velocimetry, or UADV).
However, in cases where the pregnancy is not considered high risk, it’s not recommended to do routine ultrasound tests in the third trimester.
Treatment Options for Fetal Growth Restriction
It’s crucial to identify any problems with a baby’s growth in the womb as early as possible to improve outcomes after birth. An ultrasound scan can be very useful in this regard, helping to estimate the baby’s weight and check for signs of something called Fetal Growth Restriction (FGR). FGR is when a baby is not growing at the normal rate inside the womb. During an ultrasound, measurements of the baby’s head (head circumference or HC), belly (abdominal circumference or AC), thigh bone (femur length or FL), and the distance between the two sides of the head (biparietal diameter or BD) can help detect FGR.
The size of the baby’s belly is particularly sensitive to detecting FGR. This measurement is most accurate at 34 weeks into the pregnancy or closer to the due date, especially in cases of asymmetric FGR (which is when the head and body of the fetus are not proportionate). Ratios like HC/AC and FL/HC can also provide clues on whether the baby’s growth is symmetrical or asymmetrical. Regular ultrasound scans are recommended every 3-4 weeks if FGR is suspected.
One common way to monitor pregnancies with potential FGR is to check the blood flow through the baby’s umbilical cord using a technique called Doppler velocimetry. This test helps determine the best time for delivery. If the technique detects that the blood flow through the cord slows or even reverses, then the baby may need to be delivered by 32 or 34 weeks into pregnancy respectively.
In addition to Doppler checks, monitoring techniques like cardiotocography (CTG) – which records the baby’s heartbeat and mother’s uterine contractions – and the biophysical profile (BPP) – which evaluates the baby’s movement, muscle tone, and amniotic fluid level among others, are also crucial. If the test results from these techniques are abnormal, the baby may need to be delivered through a C-section.
In situations where the baby might be born prematurely, doctors may give the mother medication to help the baby’s lungs mature more quickly. For very early births (before 32 weeks), magnesium sulfate may be given to protect the baby’s brain.
In cases of FGR detected later in pregnancy (after 32 weeks), the check-ups and monitoring are similar. Doctors may consider delivery at or past 37 weeks if any abnormality is observed in blood flow through the umbilical cord. However, it’s essential to understand that having FGR alone doesn’t necessarily mean that a C-section is required.
What else can Fetal Growth Restriction be?
When we talk about common issues with pregnancy dates and fluid volume, there are two key points:
- A pregnancy might be dated incorrectly: The most accurate way to determine the date of a pregnancy is by using a first-trimester ultrasound, which can be done either through the vagina or abdomen. Relying on the date of the last menstrual period can lead to mistakes, especially if a woman’s periods are irregular.
- Oligohydramnios, or low amniotic fluid volume: This can create a mismatch between the size of the uterus (fundal height) and the age of the pregnancy. An ultrasound scan can help predict the weight of the fetus accurately in these cases.
What to expect with Fetal Growth Restriction
The outlook varies for babies with different types of slow growth while in the womb, otherwise known as Fetal Growth Restriction (FGR). Generally, those with asymmetrical FGR, where the baby’s head and brain have grown normally while the rest of the body is smaller, have a better chance of normal growth after birth.
This is because asymmetrical FGR usually occurs later in pregnancy when the total number of body cells is normal. As a result, these babies are more likely to catch up in size after they’re born.
On the other hand, symmetrical FGR refers to the baby’s entire body being small. These infants often face an earlier complication during the pregnancy which might lead to a lower number of body cells. As a result, these babies may remain smaller in size throughout their lives.
The final height of these infants can be influenced by factors like the father’s height, mother’s height, and the length of the baby at birth. The outlook can also be different depending on the specific cause of the slow growth. For example, babies born prematurely or due to other complications face a higher risk of health issues and even death.
Possible Complications When Diagnosed with Fetal Growth Restriction
Babies who don’t grow at a normal rate in the womb (known as Fetal Growth Restriction or FGR babies) are more likely to experience both immediate and long-term health problems.
Immediate health problems may show up right after their birth. This can include issues like breathing difficulties, low blood sugar levels, thick or slow-moving blood flow, various infections, and difficulties with body temperature regulation. If these babies are born earlier than expected (premature), they may further suffer health issues related to premature birth such as intestinal disease, brain bleeding, and eye disease.
Common Immediate Health Problems:
- Breathing difficulties
- Low blood sugar (hypoglycemia)
- Thick or slow-moving blood (hyperviscosity)
- Infections (sepsis)
- Difficulties with body temperature regulation
Later on in life, babies with FGR face higher risk of death, particularly if they were very small at birth. Those that survive might not grow as tall as other kids their age. They may also have more risk of poor school performance, trouble with concentration and behaviour, and mental and motor skills development problems.
Common Long-term Health Problems:
- Poor school performance
- Concentration and behaviour problems
- Mental and motor skills development issues
Moreover, these kids are more likely to face health issues related to physical growth and development. They are at higher risk of becoming obese, getting heart diseases or diabetes, or having kidney problems as they grow older.
Preventing Fetal Growth Restriction
There have been many suggestions on how to prevent a baby from not growing enough in the womb, but only a few have shown to work. The American College of Obstetrics and Gynecology (ACOG) does not advise women at high risk of a condition called preeclampsia to take low-dose aspirin because there’s not enough proof that it makes outcomes better. However, some new studies have shown that low-dose aspirin can significantly lower the risk of preeclampsia starting early in pregnancy. In the UK, the Royal College of Obstetricians and Gynecologists does recommend low-dose aspirin before the 16th week of pregnancy for women with risk factors for preeclampsia.
It’s also important to identify as early as possible any mother who is using substances like tobacco, which can harm the blood vessels in the placenta and potentially restrict the baby’s growth. Smoking can be controlled and quitting has been proven to reduce the risk of the baby not growing enough. Advice should be offered to help mothers to stop smoking, and also for dealing with other forms of substance abuse, whenever possible.
At this time, we don’t have enough information to recommend changes in diets or suggesting bed rest to prevent poor fetal growth.