Fetal Biometry Chart by Week

A fetal biometry chart turns the four measurements on a scan report (BPD, HC, AC and FL) into a position for your baby’s exact age. The chart below gives the median and the usual range of each for every week from 14 to 40, from the INTERGROWTH-21st international standards, with every published centile for each measurement underneath.

After the chart there is a checker for your own numbers, a short guide to what each measurement is, what a low or high centile does and does not mean, and the reasons the percentile on your report may not match this page.

What are normal fetal measurements by week?

Normal fetal biometry is a range for each week, not a single number. The INTERGROWTH-21st standards, built from serial scans of 4,321 healthy, low-risk pregnancies in eight countries, give the median and the centiles of the four routine measurements from 14 to 40 weeks. At 20 weeks the median head circumference is 172.5 mm, and the 10th to 90th centile runs from 163.0 to 182.0 mm; the median biparietal diameter is 48.4 mm, the abdominal circumference 147.7 mm and the femur 31.3 mm. By 32 weeks the medians have reached 83.8 mm for BPD, 294.4 mm for HC, 273.9 mm for AC and 59.4 mm for FL. A baby whose measurements fall between the 10th and 90th centile is described as appropriate for gestational age. One outside that band is a reason to look at the other measurements, the earlier scans and the blood flow; on its own it is not a diagnosis.

Fetal biometry by week: median (10th–90th centile), mm

Source: INTERGROWTH-21st international fetal growth standards (Papageorghiou et al., The Lancet 2014;384:869–79), official centile tables © University of Oxford. Each row is an exact week: 20+0 is 20 weeks and 0 days.

Every centile, measurement by measurement

Scan reports and the definitions of growth problems also use the 3rd, 5th, 95th and 97th centiles. Each table below gives all seven published centiles for one measurement; the median is in bold.

How to check your baby’s measurements

1. Take the age on the day of the scan in weeks and days, from the date set by your early scan. Ignore the “age by biometry” printed next to each measurement: it is worked out from the measurement itself, so it cannot be used to judge it.
2. Read each measurement in millimetres. Some reports print centimetres; 17.3 cm is 173 mm.
3. Enter any of the four below. The checker interpolates the centiles for that exact day and places each measurement in its band.

For example, a head circumference of 175.0 mm at 20+0 is between the 50th and 90th centile (median 172.5 mm). Three days later, at 20+3, the median has moved on to 177.6 mm and the same 175.0 mm is between the 10th and 50th centile. The head has not shrunk; the chart has moved, which is why the checker asks for days as well as weeks.

What do BPD, HC, AC and FL measure?

All four are measured on every scan from 14 weeks onwards, which is where the chart starts; before that, the crown–rump length is used.

• BPD — biparietal diameter. The width of the head from one side of the skull to the other, taken on the same cross-section as the head circumference.
• HC — head circumference. The distance around the skull. It is less affected by the shape of the head than the BPD, and it is the measurement used to estimate the age when there was no early scan.
• AC — abdominal circumference. Around the belly at the level of the stomach and the liver. It mostly reflects the size of the liver, and with the estimated weight it is the measurement that defines a baby as small or large for its age.
• FL — femur length. The thigh bone, measured end to end.

Some reports add OFD, the length of the head from front to back, and HL, the upper arm bone. For every abbreviation on a report, including the estimated weight and the fluid measurements, see ultrasound results explained.

What does a low or high centile mean?

A centile is a position among healthy babies of the same age. The 10th centile means nine babies in ten of that age measure larger, so by definition one healthy baby in ten sits below it. Between the 10th and the 90th, a baby is described as appropriate for gestational age. With the AC or the estimated weight below the 10th centile it is small for gestational age (SGA); above the 90th, large for gestational age (LGA).

Fetal growth restriction is a different thing: a baby that is not reaching its own growth potential, usually because the placenta is struggling. FIGO’s 2021 guidance adopts the international consensus definition:

• Before 32 weeks: AC or estimated weight below the 3rd centile; or absent or reversed flow in the umbilical artery; or AC or estimated weight below the 10th centile together with raised resistance in the uterine or umbilical arteries on Doppler.
• From 32 weeks: AC or estimated weight below the 3rd centile; or at least two of these three: AC or estimated weight below the 10th centile, a fall of more than two quartiles across scans, and abnormal Doppler.

The head and femur measurements are not part of that definition, and a single AC between the 3rd and 10th centile is not enough on its own. FIGO notes that most small babies are constitutionally small and healthy. A high centile works the same way in reverse: one large measurement is a finding to follow up, and a large AC or estimated weight is a reason to check for gestational diabetes if that has not been done.

Estimated fetal weight, the number that combines all four measurements, has its own chart and a wider margin of error; our guide to fetal growth percentiles covers it.

Why one scan can mislead

The date you measure against

A centile is only as good as the age it is read at, and the chart moves quickly. Between 20+0 and 21+0 the median head circumference grows by 12.0 mm, from 172.5 to 184.5 mm, which is more than the 9.5 mm between the median and the 10th centile at 20 weeks. Read the same head against the wrong week and it changes band. That is why the checker wants the age from your dating scan, and why international guidance says that once an early scan has set the due date, later scans should not move it.

The baby’s position and head shape

Each measurement needs a precise cross-section: the head at the level of the thalami, the abdomen where the stomach and the portal vein show, the whole femur in view. A baby lying awkwardly, or a head low in the pelvis late in pregnancy, can make that plane hard to get. Head shape matters too. In a long, narrow head or a short, wide one the BPD drifts away from what is expected for the age while the head circumference stays reliable, which is why sonographers lean on the HC when the shape is unusual.

Measurement error

Even with good images, two experienced sonographers measuring the same baby agree to within 4.9% for HC, 8.8% for AC and 11.1% for FL in 95% of cases, according to an INTERGROWTH-21st reproducibility study. At 32 weeks, 8.8% of the median AC is about 24 mm, most of the distance between the 10th and 90th centile. This is why growth scans are usually spaced at least three weeks apart when the question is whether growth has changed: closer together, measurement error can pass for a change.

Which chart does your clinic use?

There is no single world chart, and the choice changes the answer. INTERGROWTH-21st is a prescriptive standard: it describes how babies grow when their mothers are healthy and well nourished, and its centiles come from 4,321 low-risk pregnancies in eight countries, each scanned every five weeks. Hadlock’s 1991 chart, one of the most widely used, comes from 392 low-risk, mostly white women in Texas. The WHO publishes fetal growth charts of its own, and many countries and hospitals use national or local ones.

They do not agree at the edges. FIGO notes that the INTERGROWTH-21st 10th centile for weight runs lower than most other ultrasound charts, so fewer babies fall below it, and suggests that universal standards may need locally adjusted cut-offs. The same measurement can therefore sit on a different centile on each of two charts without anything about the baby being different.

The BPD has one more catch. INTERGROWTH-21st measures it from the outer edge of the skull on one side to the outer edge on the other; the original method, still used with some charts, measures outer edge to inner edge. The outer-to-inner number is a little smaller for the same head, and the gap widens as the head grows — so a BPD measured that way reads slightly low against this table.

When to call your midwife

Movements are the one thing on this page you can follow yourself between scans; our guide to kick counts and baby movements explains what to watch for, and the page on blood pressure and pre-eclampsia covers the warning signs in full.

Frequently Asked Questions

Fetal biometry by week — at a glance

Fetal biometry is the BPD, HC, AC and FL measured on every scan from 14 weeks. On the INTERGROWTH-21st standard the medians at 20 weeks are 48.4, 172.5, 147.7 and 31.3 mm, and at 32 weeks 83.8, 294.4, 273.9 and 59.4 mm. Between the 10th and 90th centile a baby is appropriate for gestational age; below the 10th, small for it. Growth restriction is defined more narrowly — by FIGO in 2021, following the international consensus — through the AC or estimated weight below the 3rd centile, or below the 10th with abnormal Doppler or falling centiles. Dating, position, head shape and an observer variation of 4.9–11.1% all move a single result, and different charts place the same number on different centiles, so the trend across scans on one chart is what counts.

Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.

Read next

Sources

• Papageorghiou et al., The Lancet 2014 — International standards for fetal growth based on serial ultrasound measurements (INTERGROWTH-21st)
• INTERGROWTH-21st — Fetal growth standards: official centile tables and recommended gestational-age limits
• Melamed et al., FIGO 2021 — Best practice advice for screening, diagnosis and management of fetal growth restriction
• Salomon et al., ISUOG Practice Guidelines 2019 — Ultrasound assessment of fetal biometry and growth
• Sarris et al., Ultrasound in Obstetrics & Gynecology 2012 — Intra- and interobserver variability in fetal ultrasound measurements
• Hadlock et al., American Journal of Roentgenology 1981 — Estimating fetal age: effect of head shape on BPD
• Wong et al., Australasian Journal of Ultrasound in Medicine 2018 — Biparietal diameter measured outer-to-outer versus outer-to-inner