Growth Scans in Pregnancy

What is a growth scan?

A growth scan is an ultrasound later in pregnancy that measures your baby to check they are growing as expected. The measurements, including the abdominal circumference around the tummy, are combined into an estimated fetal weight (EFW) and plotted on a chart as a centile. ACOG, the US college of obstetricians, describes it as a specialised scan: if there are signs a baby is not growing well, its "growth rate can be tracked throughout pregnancy" this way.

One scan gives a snapshot. A series of scans gives a trend, and the guidance below puts a lot of weight on that trend.

Who gets a growth scan?

In England, not everyone. NICE guideline NG201 says: "Do not routinely offer ultrasound scans after 28 weeks for uncomplicated singleton pregnancies." Instead, growth is checked in two ways:

• A risk assessment for fetal growth restriction at your booking appointment and again in the second trimester.
• Fundal height — a tape-measure check from your pubic bone to the top of your womb — at each appointment after 24 weeks, no more often than every 2 weeks, plotted on a growth chart. If it suggests your baby is small, NICE says to offer a growth scan, more urgently if there are other concerns such as reduced movements or raised blood pressure.

The fundal height by week calculator shows roughly what the tape measure is tracking.

Reasons you may be offered planned growth scans

The Royal College of Obstetricians and Gynaecologists (RCOG) lists things that raise the chance of a small baby, including: being 40 or older; a BMI under 18.5; smoking or drug use, particularly cocaine; more than the recommended amount of caffeine; high blood pressure, diabetes, kidney disease, autoimmune disease, complicated heart disease or clotting problems; a previous small baby, pre-eclampsia or stillbirth; and heavy bleeding, especially in the second half of pregnancy. It adds that some ethnic groups (specifically Pakistani, African and Black-African Caribbean) may have an increased chance.

In England, NHS England's Saving Babies' Lives care bundle (version 3) sets out the schedule by risk group:

The bundle says scans should usually be 21 to 28 days apart while growth stays above the 10th centile, and routine growth checks should not be closer than 14 days. It also recommends a scan for growth, fluid and blood flow if you come in more than once with reduced movements after 28 weeks. Twins follow their own plan — see the twin scan schedule — and a low PAPP-A result is another common route into growth scans.

What does the centile mean?

The centile tells you where your baby's estimated weight sits among babies of the same gestation. The RCOG says a baby whose estimated weight is in "the lowest 10% of babies (the smallest 10 out of every 100 babies, or less than the 10th centile)" is considered small, and that "most babies do not have any health problems but are just smaller than others and this is normal for them".

Direction matters as much as position. The care bundle warns that a slowing growth rate — a downward trend in the centile — raises the risk of stillbirth and should prompt review, even when the baby is still above the 10th centile. You can see how measurements become a centile with the fetal growth percentile calculator and the Hadlock estimated fetal weight calculator. If your baby measures large instead, see large for gestational age.

How accurate is a growth scan?

Less accurate than the printed weight suggests. A 2005 systematic review concluded that the accuracy of EFW "is compromised by large intra- and interobserver variability" — two sonographers, or the same one on two attempts, can get noticeably different numbers. A 2018 review found ultrasound most commonly overestimated weight, that a Hadlock formula (Hadlock A) gave the most accurate results, and that measurements are harder to get right late in pregnancy and depend on the sonographer's experience.

It is still the best test available for spotting a small baby. A 2019 Cochrane review found EFW was the most accurate of the tests it compared for detecting babies who turned out small at birth. Even so, in its hypothetical group of 1,000 pregnancies containing 190 small babies, EFW would miss about 50 of them.

A growth scan also cannot re-date your pregnancy. ACOG puts third-trimester dating accuracy at plus or minus 21 to 30 days and warns that decisions based on a late scan alone are "especially problematic". Your due date comes from your early scan, as explained in the 12-week scan.

What happens if the scan shows a small baby?

It depends how small the baby is and how early it was found. The RCOG says you may be offered an umbilical artery Doppler (blood flow through the cord), more frequent growth scans, Doppler checks of blood flow in the baby's brain and abdomen, and a computerised trace of the baby's heart rate. Referral to a fetal medicine specialist is offered if your baby is very small, was found to be small early, or the umbilical artery Doppler is not normal.

On timing, the RCOG says that if your baby is small, the latest you will be advised to give birth is between 39 weeks and your due date, and that if there are other concerns about growth restriction, birth is likely to be recommended at 37 weeks. Small for gestational age goes into the tests and decisions in detail.

When to call your maternity unit

Do not wait for your next growth scan if your baby's movements change. NICE says you should contact maternity services "at any time of day or night" if you are worried about your baby's movements or notice they have reduced after 24 weeks, and the RCOG says to go straight to hospital to be checked if you are ever concerned your baby is not moving normally. Reduced fetal movements explains what happens when you call.

Frequently asked questions

How accurate are growth scans?

They give an estimate with a real margin of error. Reviews of the research find weight is more often overestimated than underestimated, and that results vary between sonographers. A 2019 Cochrane review still found the scan was the most accurate test for spotting small babies, though it would miss about 50 of 190 in a group of 1,000.

What does it mean if my baby is below the 10th centile?

The RCOG defines a baby below the 10th centile as small, and says most such babies have no health problems and are simply small. The tests that follow, especially the umbilical artery Doppler and the trend across scans, are how your team separates a healthy small baby from one that is not growing well.

Why wasn't I offered a growth scan?

In England, NICE advises against routine scans after 28 weeks in uncomplicated single pregnancies. Your growth is checked with fundal height measurements from 24 weeks instead, and a scan is offered if the measurement is small or slows, or if your risk assessment puts you in a higher-risk group.

Can a growth scan tell the sex?

That is not what it is for. ACOG says that on a standard ultrasound the sex may be visible if the baby is in a good position, but whether anyone looks for it, or tells you, at a growth scan depends on the unit. If you want to know, or want it kept quiet, say so at the start.