Growth Scan in Pregnancy: When It Is Done, What It Checks and What Your Report Means

Your doctor has written “growth scan” on your file, or you are holding a report full of letters like BPD, AC and EFW, and one number has caught your eye. Most growth scans simply show a baby growing along their own curve. Here is what the scan looks at, how to read the report, and when a result needs a closer look.

Quick Answer

A growth scan in pregnancy is an ultrasound, usually done in the third trimester, that measures your baby’s head, tummy and thigh bone to estimate weight. It also checks amniotic fluid, the placenta and the baby’s position. Weight estimates can be off by 10 to 15%, so doctors judge growth by the trend across scans, not one number.

What is a growth scan and why is it done?

A growth scan is an ultrasound that checks whether your baby is growing as expected for the number of weeks you are pregnant. Its focus is size, amniotic fluid and wellbeing, not the detailed organ-by-organ check done at the anomaly scan (often called the level 2 scan in India) around 18 to 22 weeks.

Here is where the growth scan sits among the main pregnancy scans, based on NHS guidance on pregnancy ultrasound and ISUOG growth guidelines:

Doctors ask for a growth scan for one of two reasons:

• As a routine check. Many hospitals in India schedule one growth scan in the third trimester for every pregnancy.

• Because something needs watching. You may have a health condition, a past pregnancy history or a bump measurement that raises the chance of your baby growing too slowly or too fast.

It helps to know what the evidence says. The WHO recommends one ultrasound before 24 weeks for every pregnant woman, mainly to date the pregnancy accurately. A Cochrane review of 13 trials with nearly 35,000 women found that routine ultrasound after 24 weeks in low-risk pregnancies did not improve outcomes for mothers or babies. Ultrasound has no known side effects, so a routine growth scan is not harmful, but it is perfectly fair to ask your doctor, “What are we checking for in my case?”

When is a growth scan done?

Most growth scans happen between 28 and 36 weeks of pregnancy. If you are at higher risk, scans may start at 26 to 28 weeks and be repeated every few weeks until birth.

International guidance from the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) notes that a scan at 36 weeks picked up growth restriction better than one at 32 weeks. The same guidance explains why scans are not done every week: weight estimates have a margin of error, and measuring too close together makes “slow growth” look real when it is not. Scans just 2 weeks apart gave false alarms in more than 10% of cases, so a gap of at least 3 weeks is usual unless your doctor needs to check sooner.

If you are not sure which week you are in, the Mylo due date calculator can help you match your scan dates to your pregnancy weeks.

Who may need more than one growth scan?

You may be offered repeated growth scans if something raises the chance of your baby being small or large for their age. The Royal College of Obstetricians and Gynaecologists (RCOG) lists factors such as:

• a previous baby who was small for their age, or a past pregnancy with pre-eclampsia (high blood pressure with organ strain) or stillbirth

• long-standing high blood pressure, moderate or severe kidney disease, antiphospholipid syndrome (an immune condition that affects blood clotting), or diabetes with blood vessel complications

• being 40 or older

• smoking

• heavy bleeding in the first trimester

• a low PAPP-A result (a hormone measured in the first-trimester blood test)

• a bump that is hard to measure by hand, for example with a BMI above 35, large fibroids or a lot of amniotic fluid

Many doctors also keep a closer eye on growth if you have diabetes, including gestational diabetes (high blood sugar that starts in pregnancy), because the baby may grow larger than expected. Twin pregnancies are also usually scanned more often.

What happens during a growth scan?

A growth scan is done the same way as your earlier scans. You lie on your back, gel is spread on your tummy, and a probe is moved over the skin while the sonographer takes measurements. According to the NHS, it is painless and usually takes 20 to 30 minutes, though it can take longer if the baby is in an awkward position.

A few practical tips:

• Carry your earlier scan reports. Growth is judged by comparing scans, so the old numbers matter.

• Wear loose, two-piece clothing, such as a kurta with leggings or a top with a skirt, so your tummy is easy to uncover.

• Ask the centre in advance whether you need any preparation, such as drinking water before the scan.

• If possible, have repeat scans at the same centre. Measurements vary between machines and sonographers, and staying in one place makes the trend easier to read.

How do I read my growth scan report?

The report lists a set of measurements, an estimated weight and a few wellbeing checks. Here is what the common terms mean.

A note on the weight number. Estimated fetal weight is useful but not exact. ISUOG guidance says errors of 10 to 15% are common, and the error is larger in babies who are very small or very large. So if your report says 1,800 g at 32 weeks, your baby’s actual weight could be anywhere from roughly 1,530 g to 2,070 g. This is why a single number on a single day is never the whole story.

You can read more about what is typical at this stage on Mylo’s 32 weeks pregnant guide.

My baby looks small on the scan. Should I worry?

Seeing “below 10th centile” on a report can be frightening, and many parents go straight to blaming themselves. In most cases, a baby who measures small is simply a small, healthy baby. Your doctor will look at the full picture before deciding whether there is a real growth problem.

Doctors use two different terms:

• Small for gestational age (SGA): the baby’s AC or EFW is below the 10th centile. ISUOG notes that most of these babies are “constitutionally small”, meaning small because of their genes and family build, and otherwise well.

• Fetal growth restriction (FGR): the baby is not reaching their growth potential, often because the placenta is not supplying enough. Doctors consider FGR when the baby is very small (below the 3rd centile), or when a small size comes with other signs such as abnormal Doppler blood flow or a drop across centiles between scans.

Why the chart matters for Indian mothers. The RCOG guideline lists Indian and Asian ethnicity among factors linked to babies being smaller for their age. Some hospitals use customised charts that adjust for the mother’s height, weight, number of previous babies and ethnic group, while others use a single population chart. If your baby is labelled small, it is reasonable to ask which chart was used.

What usually happens next. Your doctor may suggest a repeat scan after a few weeks to see the trend, a Doppler study, or closer monitoring of your blood pressure and the baby’s movements. If growth restriction is confirmed, the timing of birth is planned around the baby’s wellbeing. As one example, an NHS Lanarkshire guideline offers induction of labour at 39 weeks for small babies with normal fluid and Doppler, and birth at 37 weeks or earlier when growth restriction is diagnosed. Your own plan will depend on your scans and your doctor’s judgement.

Common beliefs at home, and what the evidence says

What if the scan says my baby is large?

A baby is called large for gestational age (LGA) when the EFW or AC is above the 90th centile. At full term, doctors often use the word macrosomia for a baby expected to weigh more than 4,000 g or 4,500 g.

Because weight estimates are least accurate in larger babies, one scan saying “big baby” is not a final verdict. Your doctor will consider the trend across scans, the amniotic fluid, your sugar test results if you have diabetes, and how your pregnancy is going before discussing any change to your birth plan.

When should I call my doctor before the next scan?

Do not wait for your next scheduled scan if something feels wrong. Contact your doctor or go to your maternity hospital straight away if:

• your baby is moving less than usual, you cannot feel movements, or the usual pattern has changed. The NHS advises calling immediately, even at night, rather than waiting until the next day

• you have a severe headache that does not ease with ordinary painkillers

• you notice blurred vision or flashing lights

• you have pain just below your ribs

• your face, hands or feet suddenly swell

• you have vomiting, heartburn that medicine does not relieve, or you feel seriously unwell

The last five can be signs of pre-eclampsia, which needs to be checked the same day. Avoid using a home doppler to reassure yourself: the NHS warns that hearing a heartbeat does not mean the baby is well. There is no fixed number of kicks you must count; what matters is knowing your own baby’s pattern.

Questions to ask your doctor after a growth scan

Write these down or save them on your phone before your appointment:

• Which centile is my baby on today, and how does it compare with the last scan?

• Which growth chart was used?

• Are the amniotic fluid and Doppler results normal?

• When is my next scan, and why that gap?

• Does this result change where or when I should plan to give birth?

• Which signs should make me come in before my next visit?

If you are close to term, Mylo’s 36 weeks pregnant guide covers what to prepare in the final weeks.

Frequently asked questions

Which week is best for a growth scan?

There is no single best week for everyone; it depends on why the scan is being done. For a one-time check in a low-risk pregnancy, ISUOG guidance found a scan at 36 weeks picked up late growth problems better than one at 32 weeks. If you have a risk factor, starting earlier, at 26 to 28 weeks, and repeating every 3 to 4 weeks gives your doctor a clearer trend.

Does everyone get a 32 week growth scan?

No. The WHO recommends one scan for every pregnancy before 24 weeks, but not a routine scan at 32 weeks, and a Cochrane review found routine late scans did not improve outcomes in low-risk pregnancies. Many Indian hospitals still offer a third-trimester growth scan to all patients, often somewhere between 28 and 32 weeks, while others scan only when there is a reason.

What is done in a growth scan?

The sonographer measures your baby’s head (BPD and HC), tummy (AC) and thigh bone (FL), and uses these to work out an estimated weight. They also check the amniotic fluid, where the placenta is and which way the baby is lying. If growth is a concern, a Doppler check of blood flow in the umbilical cord may be added. The whole scan usually takes 20 to 30 minutes.

Is a growth scan at 34 weeks normal?

Yes. A growth scan at 34 weeks falls well within the usual window of 28 to 36 weeks. It may be your hospital’s routine timing, or a follow-up planned 3 to 4 weeks after an earlier scan so your doctor can see how your baby’s growth is trending.

Can a growth scan change my due date?

Usually not. Your due date is set most accurately by an early scan, which is one reason the WHO recommends a scan before 24 weeks. Later in pregnancy, babies vary more in size, so a third-trimester scan is used to check growth against your existing due date rather than to reset it. You can check your dates with the Mylo due date calculator.

Why did two centres give me different weights a few days apart?

Measurements vary between sonographers and machines, and small differences in each measurement add up in the weight formula. ISUOG guidance notes this variation is large enough that scans less than about 3 weeks apart can give misleading results. Compare like with like, ideally at the same centre.

Will the growth scan tell me if it is a boy or a girl?

No. In India, the PCPNDT Act prohibits determining or disclosing the sex of the baby during pregnancy, and doctors and scan centres are bound by this law.

Is there anything I can do to help my baby grow well?

If you smoke, stopping is the lifestyle change with the clearest evidence for a baby’s growth, and your doctor can help you with support. Beyond that, keep your antenatal appointments, eat balanced meals, and stay aware of your baby’s movements so you can act quickly if anything changes.

References

1. Salomon LJ, et al. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound in Obstetrics and Gynecology, 2019.

2. Royal College of Obstetricians and Gynaecologists. The Investigation and Management of the Small-for-Gestational-Age Fetus, Green-top Guideline No. 31 (2nd edition, 2013).

3. Royal College of Obstetricians and Gynaecologists. Small-for-Gestational-Age Fetus and a Growth Restricted Fetus, Investigation and Care, Green-top Guideline No. 31 (3rd edition).

4. Bricker L, Medley N, Pratt JJ. Routine ultrasound in late pregnancy (after 24 weeks’ gestation). Cochrane Database of Systematic Reviews, 2015.

5. World Health Organization. Maternal and fetal assessment update: imaging ultrasound before 24 weeks of pregnancy. WHO antenatal care recommendations, 2022.

6. NHS. Ultrasound scans in pregnancy.

7. NHS. Your baby’s movements.

8. NHS. Pre-eclampsia.

9. NHS Lanarkshire. GAP Guidance: Assessment of Fetal Growth and Management of SGA, 2023.

10. Patnaik AM, Kejriwal GS. A perspective on the PCPNDT Act. Indian Journal of Radiology and Imaging, 2012.