How to Read a Fetal Growth Chart
A fetal growth chart shows how a baby's measurements compare with a reference population at the same gestational age. A single point shows size. Two or more points over time show growth. This guide explains what the lines on the chart mean and how to interpret serial measurements.
1. The axes
The horizontal axis is gestational age in weeks. Gestational age should come from the best available dating, usually a first-trimester crown–rump length scan, and not from the size of the baby on the current scan. The vertical axis is the measurement itself: millimetres for BPD, HC, AC and FL, and grams for estimated fetal weight (EFW).
On this site, each parameter has its own chart. Patient measurements are marked with a red cross and numbered in order, and consecutive points are joined by a line so the trend is easy to see.
2. What a centile means
A centile tells you what percentage of the reference population had a smaller value at the same gestational age. A fetus on the 25th centile for AC has an abdominal circumference larger than 25% and smaller than 75% of fetuses in the reference population at that gestation.
The charts on this site show five WHO centile lines:
| Line | Style on the chart | Meaning |
|---|---|---|
| 95th centile | Red, dotted | Only 5% of the reference population are larger |
| 75th centile | Green, dashed | Upper quartile boundary |
| 50th centile | Black, solid | Median: half larger, half smaller |
| 25th centile | Green, dashed | Lower quartile boundary |
| 5th centile | Red, dotted | Only 5% of the reference population are smaller |
The space between two lines is a centile band. In the Measurement History table, the tool labels each value with its band (for example "25–50th", or "<5th" when below the 5th centile line), using the same WHO curves drawn on the chart.
3. Interpreting a single measurement
A single scan only tells you about size at that moment. A fetus below the 5th centile may be constitutionally small and healthy, or may be growth restricted. A single measurement cannot separate the two. That is why the whole picture matters: the other biometric parameters, amniotic fluid, Doppler findings, maternal history and, most of all, the trend over time.
It also helps to look at the parameters together. A small AC with a normal HC and FL fits a different pattern from a fetus where all parameters are small in proportion. A single isolated outlier, such as one short FL with everything else normal, is often measurement variation.
4. Interpreting serial measurements
The main value of a growth chart is to show the trajectory. With two or more measurements, check whether the points:
- Track along a centile. A fetus that stays in roughly the same band over several weeks is usually growing as expected for that fetus, even if the band is low.
- Cross centiles downwards. A fall across centile lines, especially for AC and EFW, suggests that growth is slowing. In the consensus definition of late fetal growth restriction, crossing centiles by more than two quartiles is one of the criteria. The 25th, 50th and 75th centile lines on this chart are the quartile boundaries, so quartile crossing can be seen directly.
- Cross centiles upwards. Accelerating AC and EFW may suggest excessive growth, for example in poorly controlled diabetes. It may also be measurement variation.
- Plateau. A flat line between two scans, where the measurement has hardly changed, is a warning sign and should prompt careful review.
Scan interval matters. Every ultrasound measurement has some error. If two scans are too close together (less than about 2 weeks), the real growth may be smaller than the measurement error, and a false "fall" or "plateau" can appear. Follow your local protocol for scan intervals.
5. Common pitfalls
- Wrong gestational age. If the dating is wrong, every point sits in the wrong place horizontally. A fetus that is "small for dates" may simply be younger than thought. Never re-date a pregnancy from a later growth scan.
- Wrong units. Entering BPD in centimetres (4.7) instead of millimetres (47) places the point far off the chart. The tool warns you when a value is far outside the expected range.
- Mixing references. Different charts (WHO, INTERGROWTH-21st, Hadlock, local charts) give slightly different centiles. Plotting with one reference and interpreting with another causes confusion. Use one reference consistently for a pregnancy.
- Over-reading one point. One unexpected value in an otherwise steady series is more likely to be measurement variation than a real change. Repeat or review the image if in doubt.
- Forgetting the clinical picture. Liquor volume, Doppler findings, fetal movements and maternal conditions such as hypertension change how a growth pattern should be interpreted.
6. Where growth charts fit in FGR assessment
Fetal growth restriction (FGR) means a fetus that has not reached its growth potential. It is not the same as being small for gestational age (SGA). The international Delphi consensus (Gordijn et al., 2016) defines FGR using biometry together with Doppler findings:
| Early FGR (<32 weeks) | Late FGR (≥32 weeks) |
|---|---|
| AC or EFW <3rd centile, or absent end-diastolic flow in the umbilical artery or AC or EFW <10th centile combined with uterine artery PI >95th centile and/or umbilical artery PI >95th centile |
AC or EFW <3rd centile or at least two of: • AC or EFW <10th centile • AC or EFW crossing centiles by more than two quartiles • Cerebroplacental ratio <5th centile or umbilical artery PI >95th centile |
Note: this tool draws the 5th, 25th, 50th, 75th and 95th centile lines. It does not draw the 3rd or 10th centiles used in the definitions above. You can see crossing of quartiles directly on the chart, but for the 3rd and 10th centile thresholds use your unit's reference or protocol.
References
- Kiserud T, Piaggio G, Carroli G, et al. The World Health Organization Fetal Growth Charts: a multinational longitudinal study of ultrasound biometric measurements and estimated fetal weight. PLOS Med. 2017;14(1):e1002220. doi:10.1371/journal.pmed.1002220
- Gordijn SJ, Beune IM, Thilaganathan B, et al. Consensus definition of fetal growth restriction: a Delphi procedure. Ultrasound Obstet Gynecol. 2016;48(3):333–339.