Contact your maternity unit immediately — do not book a scan — if your baby’s movements have reduced or changed, if you have bleeding, if your waters break, or if you have severe headache, visual disturbance or sudden swelling. These need same-day assessment on a labour ward.
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A growth scan measures your baby’s head, abdomen and thigh bone, uses those to estimate weight, then checks the amniotic fluid and the blood flow through the umbilical cord. Together those tell you whether the baby is growing as expected and whether the placenta is doing its job. The estimated weight carries a margin of error of roughly 10 to 15 per cent, which is the single most important thing to understand before you read your report.
A growth scan — sometimes called a fetal wellbeing scan — is usually done from 24 weeks onwards. It takes around 20 to 30 minutes and needs no preparation. Four things are assessed.
Head circumference (HC) is measured around the widest part of the skull.
Abdominal circumference (AC) is measured around the tummy at the level of the liver and stomach. Of the three, this is the measurement most sensitive to how well a baby is being nourished — a baby short of nutrients tends to lay down less fat and glycogen around the liver, so the abdomen falls away before the head does.
Femur length (FL) is the thigh bone, giving a measure of skeletal growth.
Amniotic fluid and umbilical artery Doppler. Fluid volume and the pattern of blood flow through the cord both reflect placental function. A well-functioning placenta shows continuous forward flow throughout the heartbeat cycle. Changes here can be an earlier signal than size alone.
The three body measurements are entered into a validated formula — most commonly Hadlock — which produces an estimated fetal weight (EFW). That figure is then plotted on a growth chart against your gestational age, giving a centile.
A centile is simply a position in a hundred. On the 30th centile, thirty babies in a hundred at that gestation would be smaller and seventy larger. It is a description of size, not a mark out of a hundred.
Many UK units use customised charts, which adjust the expected range for your height, weight, ethnicity and number of previous babies. A tall woman and a petite woman should not be measured against the same expectation.
This is the part worth reading twice. Ultrasound does not weigh a baby. It measures three dimensions and infers weight from them, and that inference carries a margin of error of approximately 10 to 15 per cent.
In practice, an estimate of 3,000 g means the baby is most likely somewhere between roughly 2,550 g and 3,450 g. That is a wide bracket, and it is why an EFW alone is a poor basis for a big decision. What is far more informative is the trend across two or more scans — whether the baby is tracking along its own line or drifting away from it.
Some plain translations:
The pattern across scans, the Doppler and the fluid are what the clinical picture rests on — not one number in isolation.
A growth scan report comprehensively covers fetal biometry — head circumference, abdominal circumference and femur length — to calculate an estimated fetal weight, alongside amniotic fluid volume and umbilical artery Doppler flow to confirm the placenta is functioning properly.
Patients predominantly book these privately for peace of mind during the long gap between the 20-week NHS scan and birth, very often because a routine tape-measure check of the bump suggested they were measuring slightly large or small.
The single biggest patient misunderstanding is treating the estimated weight as absolute gospel. We spend a lot of time reassuring parents that ultrasound weight calculations carry a standard 10 to 15 per cent margin of error, and that babies grow in natural spurts rather than in perfect, predictable lines on a graph. A baby that appears to have stalled between two scans has very often simply been measured at two different points in its own rhythm.
Describes general patterns among the patients we see, not any individual case. Any concern about growth should be discussed with your midwife or obstetric team.
It is not a substitute for NHS antenatal care, and we report back to your midwife or obstetric team at your request.
It carries a margin of roughly 10 to 15 per cent. An estimate of 3,000 g means the baby most likely weighs somewhere between about 2,550 g and 3,450 g. The trend across scans is more informative than any single figure.
Not necessarily. Many babies on lower centiles are constitutionally small and completely well. What matters more is whether growth is tracking steadily, and what the Doppler and fluid show. Discuss the report with your midwife.
Growth scans are usually spaced at least two weeks apart, because babies need time to grow measurably. Scanning more often tends to produce noise rather than information.
From 24 weeks up to around 38 weeks. Before 24 weeks there is usually not enough of a gap since the 20-week scan for growth to be meaningfully assessed.
No. It estimates weight on the day of the scan. Babies continue to gain weight after it, and the estimate itself has a margin of error.
No. A growth scan assesses size, fluid and placental blood flow. Structural assessment is done at the 20-week anomaly scan, which is provided through NHS antenatal care.
A growth scan is £235. You receive instant verbal results at the appointment from the consultant performing your scan, with the written report following within 24 hours.
See also our presentation scan and the full range of pregnancy ultrasound scans.
CQC-registered clinic · HCPC-registered sonographers · BMUS member · ICO-registered. Reports can be shared securely with your midwife or obstetric team at your request.
Related: what actually happens at an early pregnancy scan.
