Growth scan in pregnancy: why it is offered, what happens and what results mean

A growth scan is an ultrasound scan used later in pregnancy to measure the baby and estimate their size and weight. The NHS describes ultrasound scans as using sound waves to build a picture of the baby in the womb, and says a scan can be used to check that the baby is growing normally. The measurements of the baby are combined into an estimate of size and compared with what is expected for the stage of pregnancy.

Growth scans are different from the two scans that are offered to everyone. The NHS says hospitals in England offer at least 2 scans during pregnancy, at 11 to 14 weeks and between 18 and 21 weeks, and that the 20-week scan checks for 11 physical conditions in the baby. A growth scan is an additional scan, offered when there is a reason to look more closely at how the baby is growing. In some cases it will be repeated at intervals of a few weeks so that the pattern of growth can be seen over time.

Tommy's explains that growth is checked at antenatal appointments from 24 weeks, when the midwife measures the size of the bump from the top of the womb down to the pubic bone, records it in the maternity notes and on a growth chart. If the midwife has any concerns about the baby's growth they will advise an extra ultrasound scan. Tommy's says this can be worrying but does not always mean something is wrong, and that the scan is just a more accurate way of checking the baby's size.

The NHS also lists reasons for monitoring growth, including twin pregnancies and problems in this pregnancy or a previous one. Tommy's lists factors that increase the chance of fetal growth restriction, such as a previous small baby or stillbirth, problems with the placenta, high blood pressure or pre-eclampsia in a previous pregnancy, certain health conditions including kidney disease, diabetes and autoimmune disease, smoking, using illegal or recreational drugs, being aged 40 or over, and being underweight. People with these risk factors are usually offered extra scans.

The scan is carried out by a sonographer, and it is much the same as other pregnancy scans. The NHS describes lying on your back with your tummy revealed, gel being applied and a probe passed over the tummy while the picture appears on a screen. A scan usually takes around 20 to 30 minutes, but it may take longer or have to be repeated if the baby is lying awkwardly or moving a lot.

You may also be offered Doppler measurements. Tommy's lists an umbilical artery Doppler ultrasound scan, which measures blood flow through the umbilical cord, and extra Doppler scans measuring the blood flow in the baby's brain and abdomen, a computerised cardiotocograph (a heart tracing) and an assessment of the amniotic fluid as tests that may be offered if growth restriction is suspected. At every appointment your blood pressure is checked and your urine is tested for protein.

The NHS says the sonographer can tell you the results of the scan at the time. If something looks different from expected the sonographer may ask for a second opinion, and you may be offered further tests. It is your choice whether to have scans, and your care will continue as normal if you decline.

The result is usually expressed as a centile, which shows where your baby's estimated size sits compared with other babies at the same stage. Tommy's explains that if the estimated size is in the smallest 10% (10 out of 100 babies) the baby is considered small, sometimes called small-for-gestational-age, which is not the same as fetal growth restriction. If the estimated size is in the smallest 3% or in the smallest 10% with signs that the placenta is not working well, the baby is said to have fetal growth restriction, which increases the risk of pregnancy complications.

Tommy's also says that some babies are just smaller than others, that this is normal for them and that most of these babies do not have health problems. It notes that you may have a higher risk of having a small, but not necessarily growth-restricted, baby if you are of Pakistani, African or Black-African Caribbean heritage.

The NHS adds that scans cannot pick up all health conditions, so it is important to speak to a doctor or midwife if you are worried that something is not right.

Tommy's says that if your team is concerned about how your baby is growing you will be offered closer monitoring with extra ultrasound scans, and are likely to be advised to give birth before 38 weeks. You may be referred to a specialist doctor if the baby is very small, was found to be small early in pregnancy, or if the umbilical artery Doppler is not normal. The team will talk to you about the best time for your baby to be born, which may be earlier than your due date, and about the risks and benefits of birth at that time.

Tommy's also says that people with a baby with growth restriction are advised to give birth in a hospital with a specialist baby unit, and that if your baby is likely to be born preterm you will be offered a course of steroids and, if you give birth before 34 weeks, magnesium sulphate. After birth the baby is closely monitored. You may be able to have a vaginal birth with induction, or you may be advised to have a caesarean, and your team will discuss the options.

Tommy's explains that feeling your baby move is a sign that they are well and that there is no set number of normal movements. From 16 to 24 weeks you should feel the baby move more and more up until 32 weeks, and from then movements should stay roughly the same until you give birth. Call your maternity unit and go to get checked straight away if your baby is moving less, movements feel weaker or have stopped. Do not wait until the next appointment.

Tommy's also advises contacting your GP, midwife or maternity unit if you have heavy vaginal bleeding, especially in the second half of pregnancy, because bleeding can affect your baby's growth.

On the NHS, growth scans are arranged through your midwife or maternity unit when there is a clinical reason, such as a bump measurement that is different from expected or a risk factor, and the results are linked to the rest of your maternity care. Private ultrasound providers also offer scans, sometimes including growth measurements. A private scan can be reassuring, but it sits outside your maternity record, so if a measurement worries you or differs from NHS care it is important to share it with your midwife or maternity unit promptly so that the right follow-up can be arranged.