What is Fetal Growth Restriction (FGR)?
Fetal Growth Restriction happens when a baby does not grow in the womb as expected. The most common reason is a placenta that does not give enough nutrients and oxygen to the baby. This can cause serious health problems, especially if it starts early in the pregnancy.
What does the placenta do?
The placenta is where the blood of the mother and the baby come flows very close together. This allows nutrients and oxygen to go to the baby and waste products to go to the mother. If the placenta does not werk well, the baby cannot grow properly and gets low oxygen levels.
Why is it important to find FGR early?
Diagnosing FGR early is very important. If the placenta does not work well, it can cause health problems for the baby and sometimes also for the mother. Signs of FGR are the baby being too small and abnormal blood flow in the baby and the mother. These signs can show low oxygen levels, which can be very dangerous for the baby if birth is not done quickly.
What is normal fetal growth?
Not all babies grow the same way. There is natural variation, especially in the later stages of pregnancy. Around 20 weeks of pregnancy, the average baby weighs about 300 grams. By 28 weeks, it weighs about 1200 grams, and by 32 weeks, about 1900 grams. In these 4 weeks only, babies grow nearly 60% on average! After 32 weeks, the growth rate slows down, also in normal pregnancy.
We often compare the growth of the baby to normal values. This means we plot the fetal measurements on reference curves and compare them to a large group of babies measured at the same stage of pregnancy. We use percentiles to express this. For example, a baby at the 40th percentile (p40) means that approximately 40% of babies at that gestational age are smaller, and 60% are larger. If the baby is around the same percentile in later measurements, it is likely normal.
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One way to measure the baby’s size is by feeling the height of the uterus through the abdomen. If this seems suboptimal, challenging to feel due to the mother’s weight, or if there’s an increased risk of fetal growth restriction, a growth ultrasound is typically recommended. During this ultrasound, various measurements of the head, abdomen, and legs are taken and combined to estimate the weight of the baby (called estimated fetal weight). It is important to note that these measurements are not extremely precise, so that the exact number is interpreted as an approximation rather than a scale weight. Also, with one measurement we know the baby’s size, with repeated ultrasound measurements we have an estimation of the growth. In order to assess the baby’s growth, it is best to leave ample time between measurements to account for the uncertainties of ultrasound measurements.
What is abnormal fetal growth?
To check if the growth of the baby is too slowly, we compare the baby’s size to normal values. We do this by plotting the size of the baby on reference curves and compare it to many other babies measured at the same stage of pregnancy. We use percentiles to show this. For example, a baby at the 40th percentile (p40) means that approximately 40% of babies at that gestational age weigh less, and 60% weight more. If a baby is at the 3rd percentile (p3), only 3 percent of babies are smaller.
The challenge is to find the baby who is not growing well bacause the placenta is not working properly. This is more likely at the lowest percentiles. Another clue is percentile decreases in later measurements. We also look for abnormal blood flow in the baby’s umbilical cord and brain, or in the mother’s blood vessels that supply the womb. Or if we find abnormal flow patterns in the baby’s umbilical and brain artery or in the mother’s blood vessel feeding the womb (uterine artery).
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Blood vessels are assessed with Doppler ultrasound. It shows the blood flow velocity patterns and the measurements we take reflect the resistance of the organs that the blood flows towards.
The vessels that are most often measured are:
- Maternal uterine artery: An abnormal measurement can indicate that indeed the placental development has not been optimal.
- Fetal umbilical artery: An abnormal measurement may suggest high resistance in the placenta, which may be simplified as ‘the placenta is too small for this fetus‘. This measurement therefore reflects the increased effort from the fetal heart.
- Fetal middle cerebral artery: If the fetus encounters placental insufficiency, a baby redistributes blood preferentially towards the most important organs (the brain and the heart) by reaching the resistance. This may be simplified as ‘a good thing to apply if needed, but unfortunate that apparently it is needed‘.
- Fetal ductus venosus: this is the blood vessel that transports the oxygen-rich blood from the placenta back to the heart of the baby. When the baby’s heart is struggling to perform the increased effort and starts to fail, this may be seen here.
How is FGR diagnosed?
If a baby is small, it may be just naturally small, or there could be another reason. The placenta might not be working well, or it could be due to a syndrome, a virus, a disease in the mother, smoking, anemia or poor nutrition.
Diagnosing is not always easy and your doctor will look at all these possibilities and decide which one is most likely.
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When your doctor looks at all the options, there is usually no single tool that gives a clear answer right away. Instead, doctors look at patterns to recognize by checking all signs and symptoms toghether. Each individual situation needs a special approach.
- Constitutionally small: among all small babies (the smallest 10 percent), this is the most common reason. But it is ultimately the likely diagnosis only after excluding other causes.
- Placenta: Mostly indicated if the blood flow between the mother and baby is abnormal or if the baby’s growth is slow.
- Syndrome-related anomaly: if the baby is small for gestational age, this should also be considered. Tests to assess this are an anomaly scan, non-invasive prenatal testing (NIPT) or amniocentesis.
- Viral illnesses: Some viruses like cytomegalovirus, parvovirus, can cause abnormal growth. Recent infections can be tested in the mother’s blood, especially if there are signs of echogenic bowel or calcifications in the brain.
- Maternal disease: Some diseases in the mother can affect blood flow to the womb.
- Smoking: If a pregnant woman smokes, it can cause low oxygen levels for the baby.
- Anemia: Severe and prolonged anemia can affect oxygen transport to the womb.
- Malnourishment: If a pregnant woman has severe dietary deficiencies, it can affect the baby’s growth.
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