Fetal growth changes quickly during pregnancy, so the measurements seen on an ultrasound at 20 weeks will be very different from those at 32 or 38 weeks. Fetal weight and length-related measurements help doctors assess this growth and check whether the baby is developing as expected for the gestational age.
The WHO fetal growth charts provide reference values for estimated fetal weight (EFW) and several ultrasound measurements from 14 to 40 weeks. Ultrasound-derived weight is an estimate, and doctors interpret it along with the baby's growth pattern and other pregnancy findings.
The following table uses the WHO median estimated fetal weight for each completed week from 14 to 40 weeks. WHO uses specific ultrasound measurements such as head circumference, abdominal circumference and femur length.
During weeks 8-13, fetal length is measured from the top of the head to the bottom (Crown-Rump Length or CRL). From week 14 onward, ultrasound scans measure individual body biometric markers such as Femur Length (FL) rather than total length; head-to-heel length values shown after 14 weeks are approximate conversions.
The increase becomes particularly noticeable during the second half of pregnancy. The WHO median EFW rises from about 0.31 kg at 20 weeks to 1.19 kg at 28 weeks and 2.75 kg at 36 weeks. By 40 weeks, the median is about 3.62 kg.
At this stage, doctors usually focus on confirming gestational age and evaluating early fetal development. Estimation of fetal weight in kilograms is not recommended at this stage. Crown-rump length helps to accurately determine pregnancy dating during the first trimester.
During the second trimester, the weight of the fetus can be measured using ultrasound assessments. Doctors calculate Estimated Fetal Weight (EFW) using the fetal head circumference, abdominal circumference, and femur length measurements. Fetal growth becomes progressively faster as pregnancy advances.
Weight gain continues rapidly during the third trimester. A growth scan may assess EFW along with abdominal circumference, amniotic fluid, and Doppler assessment of placental or umbilical blood flow when recommended by the doctor. All of these measurements are considered for fetal growth assessment.
Pregnancy is dated in weeks rather than calendar months. The month-wise view below is a simple way to understand the progression but it should not replace the gestational age written on an ultrasound report.
A growth ultrasound does not weigh the fetus directly. Instead, measurements of the baby's body are used to calculate an estimated fetal weight.
The measurements that are commonly assessed are:
It is important to note that EFW is not the same as birth weight. It can differ from the weight measured after delivery because it is calculated from ultrasound measurements.
First, you should find the gestational age (e.g., 20 weeks), then look at the estimated fetal weight and its percentile. A percentile will show where the measurement falls compared with the reference population at the same gestational age. For example, if your baby is in the 50th percentile, it means their weight is right in the middle compared to 100 healthy babies at the same stage.
Most importantly, doctors look for steady growth over time across the scans. Thus, being slightly below or above the median average does not mean there is a problem, as every healthy baby grows at their own natural pace.
A lower EFW can result from differences in dating, normal constitutional size or a condition affecting fetal growth. Placental dysfunction, maternal health conditions and fetal factors can contribute to fetal growth restriction.
Most doctors use combinations of EFW or abdominal circumference, gestational age and Doppler findings when assessing suspected fetal growth restriction rather than relying on weight alone.
A higher EFW can occur because of normal variation, parental characteristics, maternal diabetes or other factors. The treating obstetrician considers the percentile, abdominal circumference, pregnancy history and other scan findings before deciding whether additional monitoring is needed.
Fetal growth can be influenced by:
Maternal food intake alone does not determine fetal size. Growth reflects several maternal, placental and fetal factors.
A doctor may recommend closer assessment when fetal measurements fall below or above expected percentiles, growth appears to slow between scans, or there are concerns about the placenta, amniotic fluid or fetal blood flow.
Depending on the findings, follow-up may include a repeat growth ultrasound, Doppler assessment of blood flow, amniotic-fluid assessment or other antenatal monitoring. Cloudnine's existing pregnancy-scan guidance also describes repeat scans and Doppler monitoring when clinical concerns arise.
Cloudnine's approach to fetal growth assessment looks beyond a single estimated weight. Growth scans can bring together fetal biometry, growth trajectory, amniotic-fluid assessment, placental findings and Doppler blood-flow assessment when clinically required. This helps the obstetric team distinguish a normal variation in fetal size from findings that need closer monitoring.
A fetal weight chart is most useful when it helps put an ultrasound measurement into context. The WHO fetal-growth standards provide reference values for estimated fetal weight and key ultrasound measurements from 14 to 40 weeks, but an individual baby's growth is assessed through percentiles, measurements and changes over time. If an ultrasound shows a weight above or below the median, the number itself does not give the complete picture. Discuss the percentile, growth trend and other scan findings with an obstetrician so that any needed monitoring can be planned appropriately.
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Ultrasound machines calculate estimated fetal weight using measurements such as head circumference, abdominal circumference and femur length. To convert grams to kilograms, divide the value by 1,000. For example, 1,901 g is 1.90 kg.
The WHO median EFW is approximately 476 g at 22 weeks and 565 g at 23 weeks. A fetal weight of 500 g therefore falls between the median values for these two weeks.
The WHO median EFW at 32 weeks is about 1.90 kg. An estimate of 2.1 kg is above the median, but the percentile and other ultrasound findings are needed to interpret the measurement properly.
The WHO median EFW at 34 weeks is about 2.31 kg. An estimate of 2.2 kg is close to the median and should be interpreted alongside the baby's percentile and growth trajectory.
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