Almost everyone leaves a scan holding a printout they cannot read. Two columns of three-letter abbreviations, a number next to each, sometimes a percentile, and a line at the bottom giving a weight nobody has weighed. Nothing on it is explained, and the appointment where it might have been is already over.
This page is the translation. Every abbreviation you are likely to see, what each measurement actually is, how the estimated weight is produced and how far out it can be, and which lines on the report change what happens next.
The short answer: BPD is head width, HC head circumference, AC abdominal circumference, FL femur length and CRL crown-rump length. EFW is the estimated fetal weight, calculated from the others rather than measured — in one term study only 56.5% of estimates were within 10% of actual birth weight. AFI is the amniotic fluid index, normally 5 to 24 cm. A percentile is a position against a reference population, not a grade.
Every abbreviation on the report
| Code | Full name | What it is |
|---|---|---|
| CRL | Crown-rump length | Head to bottom of spine. The dating measurement, used to about 14 weeks |
| BPD | Biparietal diameter | Width of the head between the parietal bones |
| HC | Head circumference | Around the head. More reliable than BPD when the head shape is unusual |
| AC | Abdominal circumference | Around the belly at liver level. The measurement most sensitive to growth problems |
| FL | Femur length | The thigh bone |
| HL | Humerus length | The upper arm bone. Not always measured |
| EFW | Estimated fetal weight | Calculated from HC, AC, FL and often BPD. Not a measurement |
| AFI | Amniotic fluid index | Four pockets of fluid added together, in centimetres |
| DVP / MVP | Deepest vertical pocket | The single largest pocket of fluid. An alternative to AFI |
| FHR | Fetal heart rate | Beats per minute at the time of the scan |
| PI / RI | Pulsatility / resistance index | Doppler measures of blood flow resistance, usually in the umbilical artery |
| GA | Gestational age | Age by biometry — the scanner's estimate from size, not your dates |
| EDD | Estimated due date | Set in the first trimester and not normally revised afterwards |
Estimated fetal weight, and how wrong it can be
EFW is the line everyone reads first and the one that deserves the most caution. The scanner does not weigh the baby. It measures head, abdomen and femur, and puts those into a formula — usually one of Hadlock's, published in the 1980s and still the standard.
Each input carries a measurement error, and the formula multiplies them together. The result is a number that looks precise and is not.
📊 How often the estimate is close
- 56.5% of estimates fell within 10% of actual birth weight in one term study
- 72.2% fell within 15%
- 80.5% fell within 20%
- Analyses of the Hadlock formulae put the 95% limits of agreement at around 20% either way
Concretely: an EFW of 3,000 g at term is consistent with a baby anywhere from roughly 2,400 g to 3,600 g. That is not a failure of the scan — it is the known precision of the method, and it is why decisions are rarely made on a single estimate.
One scan is a dot; two scans are a line. A single EFW says relatively little. Two or three, plotted over several weeks, show whether the baby is following its own curve — which is the actual question a growth scan is asked. If you are having serial scans, ask for each estimate rather than only the latest one.
Amniotic fluid
The AFI is measured by dividing the uterus into four quadrants, finding the deepest pocket of fluid in each and adding them up. A normal AFI in a singleton pregnancy runs from 5 to 24 cm.
- Oligohydramnios — too little fluid. Traditionally an AFI of 5 cm or less, though ACOG supports using a deepest vertical pocket of 2 cm or less instead, which produces fewer unnecessary interventions.
- Polyhydramnios — too much. Graded mild at an AFI of 24.0–29.9 cm, moderate at 30.0–34.9 cm and severe at 35 cm or more.
Both are findings rather than diagnoses. Each has several possible causes, some of which need investigating and some of which do not, and both are usually followed with repeat scans rather than acted on immediately.
Percentiles and what they do not mean
A percentile places a measurement against a reference population at the same gestational age. The 50th is the median. The 10th means nine babies in ten of that age measure larger.
Two things about them are consistently misread. A low percentile is not a diagnosis: small parents have small babies, and a baby tracking steadily along the 8th percentile is usually a small normal baby. What matters is the trajectory — a baby crossing downwards through percentiles between scans is a different and more significant finding than one sitting low from the start.
Which chart was used also matters. WHO and INTERGROWTH-21st standards describe how babies grow under optimal conditions; some local charts describe how babies in that population did grow. The same measurement can land on different percentiles depending on which is used. Our guide to fetal growth percentiles works through this in detail.
Doppler measurements
If your report includes PI or RI values, blood flow has been assessed. The umbilical artery is the usual site: high resistance suggests the placenta is working harder than it should to deliver blood, which is one of the earliest signals of placental insufficiency.
Some reports add the middle cerebral artery, and sometimes a cerebroplacental ratio between the two. These are specialist measurements, they are interpreted against their own gestational-age percentiles, and they are the part of the report least worth trying to interpret alone.
Why the scan gives a different gestational age
Reports often show something like "34+2 by biometry" when you are 36 weeks. This is a common source of alarm and usually means something ordinary.
The scanner calculates an age from the measurements alone, as if the dates were unknown. That figure is used to set your due date only in the first trimester, when embryos grow at nearly identical rates. After about 14 weeks, normal variation in size is large enough that dating by measurement would be less accurate than the date you already have — so the due date does not change, and a biometric age below your actual gestation is simply a statement that the baby measures on the smaller side.
What to ask about a report
- Which percentile, and on which chart? The chart used changes the answer.
- How does this compare to the last scan? The trend is the information.
- Was the fluid measured as AFI or deepest pocket? They have different thresholds and are not interchangeable.
- Does anything here change the plan? Most reports do not, and hearing that explicitly is worth more than any single number.
When to call your midwife
Call the same day if: your baby's movements have slowed or changed after 24 weeks, you have vaginal bleeding, a gush or trickle of fluid, severe abdominal pain, a severe headache with visual disturbance, or sudden swelling of the face and hands. A reassuring scan report does not change this — reduced movements after a normal growth scan still need reporting the same day.
A scan describes one moment. Movements describe now, and they are the only thing on this page that you can monitor yourself — our guide to kick counts and baby movements covers what to watch for, and the full schedule of tests and scans covers where each report fits in the sequence.
Frequently Asked Questions
What do BPD, HC, AC and FL mean on an ultrasound report?
They are the four standard biometry measurements. BPD is the biparietal diameter, the width of the head between the two parietal bones. HC is the head circumference. AC is the abdominal circumference, measured at the level of the liver and stomach. FL is the femur length, the long bone of the thigh. Together they are used to estimate the baby's weight and to date the pregnancy.
What does EFW mean?
Estimated fetal weight — a calculated number, not a measured one. The scanner does not weigh anything; it puts HC, AC, FL and often BPD into a formula, usually one of Hadlock's. Because it is derived from four measurements, each with its own error, the result carries more uncertainty than a single number suggests.
How accurate is estimated fetal weight?
Less accurate than most people assume. In one term study, 56.5% of estimates fell within 10% of the actual birth weight, 72.2% within 15% and 80.5% within 20%. Analyses of the Hadlock formulae put the 95% limits of agreement at around 20% in either direction. On a 3 kg baby that is a plausible range of roughly 2.4 to 3.6 kg. This is why a trend across scans matters more than any single estimate.
What is a normal amniotic fluid index?
An AFI between 5 and 24 cm is generally considered normal in a singleton pregnancy. Below that is oligohydramnios — traditionally an AFI of 5 cm or less, though ACOG supports using a deepest vertical pocket of 2 cm or less instead. Above 24 cm is polyhydramnios, graded mild at 24.0–29.9 cm, moderate at 30.0–34.9 cm and severe at 35 cm or more.
What does CRL mean and why does it stop being used?
Crown-rump length, the measurement from the top of the head to the bottom of the spine. It is the most accurate way to date a pregnancy and it is used until about 14 weeks. After that babies start to curl and stretch differently and the measurement loses precision, so dating switches to head circumference and femur length.
What does the percentile on my report mean?
It places your baby's measurement against a reference population at the same gestational age. The 50th percentile is the median; the 10th means nine out of ten babies that age measure larger. A percentile is a position, not a verdict — a consistently small baby following its own curve is usually normal, while a baby crossing downwards through percentiles is what prompts a closer look.
What is the umbilical artery Doppler?
A measurement of blood flow resistance in the cord, reported as a PI (pulsatility index) or RI (resistance index). It assesses how well the placenta is delivering blood, and it is the reason a growth scan sometimes takes longer than expected. It is normal for the numbers to be reported alongside a percentile of their own, because expected resistance falls as pregnancy advances.
Why does the report list a different gestational age than my dates?
Because the scanner calculates one from the measurements, independently of your dates. That figure is used for dating only in the first trimester; after about 14 weeks your due date does not change, and the scan-derived age is simply a description of size. A report saying "34+2 by biometry" at 36 weeks does not mean your dates were wrong — it means the baby measures smaller than average, which is a different statement.
Reading an ultrasound report — at a glance
BPD, HC, AC and FL are the four biometry measurements; CRL is the first-trimester dating measurement. EFW is calculated from the others rather than measured, and its precision is limited: in one term study 56.5% of estimates were within 10% of actual birth weight, 72.2% within 15% and 80.5% within 20%. AFI is the amniotic fluid index and normally runs 5 to 24 cm, with oligohydramnios below that and polyhydramnios above 24 cm in graded bands. Percentiles are positions against a reference population, and the trajectory across scans matters more than any single value. Doppler PI and RI describe blood flow resistance. A gestational age quoted "by biometry" is a description of size, not a revision of your due date.
Inside Baby Novum: the ultrasound log stores estimated fetal weight, fetal length, head circumference and abdominal circumference for each scan with the date, gestational week, the doctor's comment and a photo of the report. Each measurement is plotted against WHO / INTERGROWTH-21st percentile bands (P5 / P50 / P95) with the FIGO 2021 mean overlaid, and the growth chart appears from the second record — so the trend this page keeps insisting on is the thing you actually see.
Read next
Sources
- StatPearls — Sonographic Evaluation of Amniotic Fluid
- Percent error of ultrasound examination to estimate fetal weight at term in different categories of birth weight
- Anderson et al., Ultrasound in Obstetrics & Gynecology 2007 — Sonographic estimation of fetal weight: bias, precision and consistency across 12 formulae
- ACOG Committee Opinion 828, 2021 — Indications for Outpatient Antenatal Fetal Surveillance
- NHS — Ultrasound scans in pregnancy