Fetal weight by week is an estimate of how much a fetus may weigh at a given stage of pregnancy, usually calculated from ultrasound measurements of the head, abdomen, and thigh bone. The figures are guides, not exact weights or pass/fail targets. Babies naturally differ in size, and an ultrasound estimate can be off. Clinicians interpret the weight alongside gestational age, growth over time, and other scan findings.
How is fetal weight estimated during pregnancy?
A fetus is not weighed directly before birth. Instead, an ultrasound scan measures parts of the body, and a mathematical formula uses those measurements to calculate estimated fetal weight, often shortened to EFW. The American College of Obstetricians and Gynecologists (ACOG) explains that ultrasound can estimate fetal size and weight while also providing information about development, the position of the fetus, the placenta, and the amount of amniotic fluid. ACOG's guidance on ultrasound exams describes these uses.
Common measurements include head circumference, abdominal circumference, and femur length, which measures the thigh bone. The head and abdomen measurements help describe body size, while the thigh bone measurement provides another indicator of growth. The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) notes in its practice guidelines for fetal measurements and growth that errors of approximately 10% to 15% in estimated fetal weight are common. This means an estimate can differ noticeably from the fetus's actual weight.
Fetal weight by week: Reference estimates from 14 to 40 weeks
The table below shows the middle estimated fetal weight for each completed week of pregnancy, from a World Health Organization (WHO) multinational study. These figures represent the 50th percentile, or the midpoint of the reference group, for pregnancies with one fetus. They are reference estimates rather than required weights for an individual pregnancy. The WHO fetal growth chart study also found differences in growth measurements across populations, so the reference chart used by a maternity team may produce somewhat different estimates.
| Completed pregnancy week | Middle estimated fetal weight (grams) |
|---|---|
| 14 weeks | 90 g |
| 15 weeks | 114 g |
| 16 weeks | 144 g |
| 17 weeks | 179 g |
| 18 weeks | 222 g |
| 19 weeks | 272 g |
| 20 weeks | 330 g |
| 21 weeks | 398 g |
| 22 weeks | 476 g |
| 23 weeks | 565 g |
| 24 weeks | 665 g |
| 25 weeks | 778 g |
| 26 weeks | 902 g |
| 27 weeks | 1,039 g |
| 28 weeks | 1,189 g |
| 29 weeks | 1,350 g |
| 30 weeks | 1,523 g |
| 31 weeks | 1,707 g |
| 32 weeks | 1,901 g |
| 33 weeks | 2,103 g |
| 34 weeks | 2,312 g |
| 35 weeks | 2,527 g |
| 36 weeks | 2,745 g |
| 37 weeks | 2,966 g |
| 38 weeks | 3,186 g |
| 39 weeks | 3,403 g |
| 40 weeks | 3,617 g |
How to use the table: Compare these numbers with the general growth pattern rather than treating them as exact targets. For example, the table gives a midpoint estimate of 1,189 grams at 28 completed weeks. A result that differs from this figure does not, by itself, establish that a fetus is growing too slowly or too quickly. The actual scan report, the chart used, and the pregnancy's overall findings provide the relevant context.
What does the fetal weight percentile mean?
A percentile compares an estimated fetal weight with estimates from a reference group at the same gestational age. The 50th percentile is the midpoint: approximately half of the reference group has a lower estimate and half has a higher estimate. A percentile is a comparison, not a prediction of a baby's future health or a guarantee of birth weight.
The International Society of Ultrasound in Obstetrics and Gynecology explains that an estimated fetal weight below the 10th percentile may be described as small for gestational age. However, a fetus can be naturally small without having a growth problem. Clinicians consider other findings, including changes in growth over time and, when indicated, blood flow through the umbilical artery, before interpreting a potentially low growth estimate. ISUOG's patient information on fetal growth restriction explains this distinction.
Why can an ultrasound weight estimate differ from the actual weight?
Estimated fetal weight is calculated from body measurements rather than obtained from a scale. Small differences in the measurements can affect the final calculation, and the amount of uncertainty can vary from one scan to another. The commonly reported 10% to 15% estimation error is a general limitation, not a precise correction that can be applied to every result.
Estimates can also differ between growth charts and calculation methods. Some charts are based on different study populations, and fetal growth is not identical across all pregnancies. A result may therefore differ from a pregnancy app, an online chart, or an estimate from an earlier scan without automatically indicating a problem.
Why does growth over time matter more than one estimate?
A single scan gives a picture of estimated size at one point in pregnancy. When additional scans are clinically appropriate, comparing measurements over time can help the maternity team understand the growth pattern. A change in the pattern may be more informative than the result of one scan alone, especially when the estimated weight and abdominal measurements are considered together.
Depending on the findings, a clinician may also review the accuracy of the pregnancy dates, the amount of amniotic fluid, the placenta, and blood-flow measurements. Maternal medical history and other pregnancy information may also affect how the findings are interpreted. Further monitoring is based on the complete clinical picture rather than on a single weight figure.
Does a smaller or larger estimate always mean something is wrong?
No. Some fetuses are naturally smaller or larger than the middle reference estimate. A difference may also reflect uncertainty in the ultrasound calculation or the growth chart being used. In other cases, a consistently unusual growth pattern can prompt an assessment for a possible growth concern. The number alone cannot distinguish these explanations.
If the maternity team raises a concern about growth, ask what the measurements show, whether they have changed from previous scans, and whether any additional monitoring is recommended. This gives you a clearer understanding of what the findings mean in your particular pregnancy without relying on a general online chart to interpret an individual result.
Can fetal weight by week predict birth weight?
An estimated fetal weight is not the same as a baby's weight at birth. The estimate describes the fetus at the time of the scan, while growth continues during the remaining pregnancy. The timing of delivery and the growth that occurs after the scan affect the eventual birth weight. The National Institutes of Health's Eunice Kennedy Shriver National Institute of Child Health and Human Development notes that factors beyond the measurements used in its fetal growth calculator can affect an infant's birth weight.
For this reason, an ultrasound estimate is useful for assessing growth and planning care when needed, but it cannot promise an exact birth weight. Your maternity team can explain how much confidence to place in the estimate and whether it has any implications for pregnancy care or delivery planning.
Are fetal weight estimates checked every week?
No. A week-by-week reference table does not mean that an ultrasound is needed every week. Scan schedules vary according to the healthcare system and individual pregnancy needs. The UK National Health Service (NHS), for example, describes routine ultrasound appointments in England at 11 to 14 weeks and 18 to 21 weeks, with additional scans offered when appropriate. These appointments have different purposes, and not every routine scan includes a detailed assessment of fetal weight. The NHS guide to ultrasound scans in pregnancy explains when scans are offered and what they can assess.
When an additional growth scan is recommended, ask what the team is monitoring and when the results will be reviewed. The purpose is to assess growth and overall pregnancy findings in context, not to make every estimated weight match a reference number.