Fetal Weight Percentile Explained

To better understand how doctors assess fetal growth during pregnancy, it’s important to first define what a percentile actually is.

When you are about to become a parent, the first thing you worry about is your baby’s health. A percentile is a measurement that allows you to track a child’s growth on a chart. During pregnancy, it enables you to monitor the fetus’s development using fetal weight percentile, which compares the baby’s growth with standardized reference curves.

The estimated fetal weight percentile is especially useful because it provides doctors with a clear indication of whether fetal growth is progressing normally. In this guide, we will explain:

• What percentiles mean;
• How doctors calculate them; and
• How to interpret the results using a fetal weight percentile chart by week.

Ready! Let’s dive in!

To accurately calculate fetal growth, healthcare professionals rely on ultrasound measurements.

Using ultrasound images, an expert (gynaecologist, paediatrician, sonographer, etc.) measures the head circumference, biparietal diameter, femoral length, and abdominal circumference. The result is then compared to the percentage obtained by the majority of babies of the same age and gender. These values serve as the basis for fetal weight percentiles and are published annually by the World Health Organisation (WHO). The average is 50.

Example

A fetus in the 50th percentile is in the “perfect” average range. This result means that out of 100 fetuses at the same gestational age, 50 are smaller than it and 50 are larger.

To better visualize these measurements, parents can consult their baby’s medical records. That’s where you’ll find the curves showing the growth of the fetus or your child, often presented as a fetal weight percentile chart by week.

Once measurements are calculated, the next step is understanding what the numbers actually mean. Between the 10th and 90th percentiles, the baby’s size is within the normal range. On a fetal weight percentile chart by week, this range represents healthy fetal growth for most pregnancies. Doctors usually focus on the evolution of the fetal weight percentile over time rather than on a single isolated value.

When the baby is below the 10th percentile

When the measurements fall below the normal range, closer medical attention may be required. When the baby is below the 10th percentile, this is referred to as “small for gestational age” or “SGA”. In this case, the fetal weight percentage raises the possibility of growth problems and usually prompts closer monitoring.

The mother-to-be must undergo several tests, including blood tests, monitoring, and more detailed ultrasounds. This is to determine whether the baby is constitutionally “small” or shows signs of fetal growth restriction (FGR), such as abnormal blood‑flow Dopplers or falling off their previous growth curve. This situation requires close monitoring from detection through delivery, which may be premature at times.

If certain isolated measurements are below the 10th percentile (head circumference, femoral length, etc.), you can seek the opinion of a specialist who will perform additional examinations to rule out genetic or infectious causes. However, an isolated low fetal weight percentile does not necessarily indicate a problem.

When the baby is above the 90th percentile

In contrast, measurements that exceed the normal range also require careful evaluation. When the baby is above the 90th percentile (a.k.a., LGA, “large for gestational age”), experts often consider screening for gestational diabetes with an oral glucose tolerance test.

If gestational diabetes is poorly controlled, the mother’s blood sugar level increases. This sugar reaches the fetus through the umbilical cord and accelerates growth, potentially raising the fetal weight percentile beyond the normal range.

The main risks are during delivery, as there is a possibility of maternal-fetal disproportion. In such cases, a cesarean section may be necessary. However, you must remember that these are only estimates, and the margin of error for the estimated fetal weight percentile based on ultrasound can exceed 20%, especially later in pregnancy.

There may also be a higher risk of postpartum haemorrhage in mothers who give birth to large babies. That’s because the uterus is more distended before birth, and preventive treatment may be administered.

Effects on the baby after birth

Beyond pregnancy and delivery, fetal growth percentiles can also influence newborn care. As for the baby, there is a risk of hypoglycemia in cases of high percentiles for fetal weight. After birth, the sudden interruption of the sugar supply from the umbilical cord can lead to low blood sugar levels. For this reason, hospital protocols include blood sugar monitoring during the first 24 hours.

As with small-for-gestational-age fetuses, a baby may have an estimated fetal weight percentile above the 90th percentile and have no associated maternal diabetes. It may simply be constitutional.

Even after delivery, growth patterns are closely monitored. If there is a break in the curve, the baby’s diet will be reassessed with the parents. If the curve is completely abnormal, the doctor will look for metabolic causes or allergies.

It should be noted that babies who are hypotrophic in utero often show catch‑up growth after birth, both physically and developmentally, although some remain smaller or need long‑term follow‑up. For macrosomic babies, many follow a stable growth curve after birth. Still, they can have a higher long‑term risk of overweight or metabolic problems, especially when maternal diabetes was present in pregnancy.

Tracking growth with a fetal weight percentile chart by week before birth and regular measurements afterward helps ensure your baby’s development stays on the right track.