Having a small baby

If the estimated weight of your baby by ultrasound scan is in the lowest 10% of babies (the smallest 10 out of every 100 babies, or less than the 10th centile), they are considered to be small.

There are different reasons why your baby may be small. Most babies do not have any health problems but are just smaller than others and this is normal for them.

However, sometimes babies are small because they have not grown as well as expected. This is called being ‘growth restricted’. The smaller your baby is, the more likely they are to be growth restricted. Causes of growth restriction include:

• Your placenta not working as well as it should – this could be because of medical problems such as having high blood pressure, or complications of pregnancy such as pre-eclampsia. You can find out more about pre-eclampsia from the RCOG patient information Pre-eclampsia.
• Infection: If you catch certain infections (such as toxoplasmosis or CMV) while you are pregnant it can sometimes affect the growth of your baby
• Having a baby with an underlying developmental or genetic condition.

If your baby is small and healthy, they will have a lower chance of complications than if they are small and are known to have an underlying health problem.

The earlier in pregnancy and the more severely your baby’s growth is affected, the more likely it is that they will have complications. Babies whose growth is only affected later in pregnancy are less likely to have severe complications.

If your baby is growth restricted, they have a higher chance of being stillborn (dying before birth), being unwell at birth or dying shortly after birth. They may also need to be born prematurely (less than 37 weeks) or to be cared for in the neonatal unit.

There are many different reasons why you might have an increased chance of having a small baby, including:

• If you are aged 40 or more
• If you are underweight (your BMI is less than 18.5)
• If you are a smoker or misuse drugs (particularly cocaine) during pregnancy
• If you have more than the recommended amount of caffeine during pregnancy
• If you have certain health conditions, such as high blood pressure, diabetes, kidney disease, auto-immune disease, complicated heart disease or blood clotting problems.
• If you have had a previous small baby
• If you have had a previous pregnancy complicated by blood pressure problems or pre-eclampsia
• If you have had a previous stillbirth.

Heavy vaginal bleeding, especially in the second half of pregnancy, can also affect the way your baby grows.

Some ethnic groups (specifically Pakistani, African and Black-African Caribbean) may have an increased chance of having a small baby.

Your health care professional will discuss the other things that can affect the growth of your baby with

• Stopping smoking is one of the best things you can do for your health and your baby’s health. Your healthcare professional can help you find support to stop smoking. If you can stop smoking by the time you are 15 weeks pregnant your chance of having a small baby will reduce to being similar to that of a non-smoking woman. You should be offered professional support to help you to stop.
• Do not use recreational drugs (see NHS information keeping well in pregnancy).
• Reduce your caffeine intake - (the current recommended maximum intake is 200mg per day, for example, two mugs of instant coffee)
• Maintain a healthy weight (see RCOG patient information Healthy eating and vitamin supplements in pregnancy).

If you are at increased risk of having problems with your placenta or of developing pre-eclampsia, you should be advised to take low-dose aspirin (150 mg once each evening) from 12 weeks of pregnancy until 36 weeks of pregnancy.

Your healthcare team will check your risk of having a small baby at your first booking appointment and as your pregnancy progresses:

• If you have a lower chance of having a small baby, your baby’s growth will still be monitored:

At each antenatal appointment, from 24 weeks of pregnancy onwards, the distance between your pubic bone and the top of your uterus (womb) should be measured. This is called the symphysis fundal height (SFH) measurement and it will be plotted on a chart. Recording this measurement should give reassurance that your baby is growing as expected.

The distance between the top of your uterus and your pubic bone will be measured to check the growth of your baby.

If the growth slows down or the measurement suggests that your baby may be small, you will be advised to have an additional ultrasound scan.

• If you have a higher chance of having a small baby, you will be offered a referral for either:
  • Additional ultrasound scans to measure your baby’s growth– how often you have these growth scans will depend on your individual circumstances.
  • A uterine artery Doppler scan – this is an ultrasound scan of the blood flow to your placenta. This is usually done at around 20 weeks when you have your routine anomaly scan. Depending on the results, you will be advised how often you need further growth scans to measure the size of your baby.

What happens will depend on how small your baby is and how early in the pregnancy your baby has been found to be small. You may be offered the following tests to check your baby’s wellbeing:

• An umbilical artery Doppler scan – this is an ultrasound scan which measures the flow of blood through the umbilical cord. It can help to tell whether your baby is at risk of becoming unwell and whether they may need to be born early
• More frequent growth scans
• Additional Doppler scans checking the blood flow in the baby’s brain and abdomen
• A computerised cardiotocograph (cCTG) – this is a tracing of your baby’s heart rate.

You may be referred to a specialist doctor (a fetal medicine specialist) for more frequent and detailed scans if:

• your baby is very small,
• they have been found to be small early in the pregnancy, or,
• the umbilical artery Doppler scan is not normal.

Depending on your individual circumstances you may be offered additional investigations. For example, you may be offered a test to check whether your baby has any genetic or chromosomal conditions (an amniocentesis), or blood tests to check for infections.

Your blood pressure and urine will be checked regularly to see if you are developing pre-eclampsia.

You will be advised to go straight to hospital to be checked if you are ever concerned that your baby is not moving normally.

This will depend on your individual circumstances. The latest you will be advised to give birth if your baby is small is between 39 weeks and your due date (which is 40 weeks). If there are other concerns that your baby is growth restricted, you will be advised to give birth sooner. An individual plan of care will be made with you depending on your circumstances but birth is likely to be recommended at 37 weeks.

Sometimes if your baby is very small or if the umbilical artery Doppler measurements are not normal, you may be advised to give birth prematurely. Your healthcare team will discuss the risks and benefits of this with you.

You will be advised to give birth to your baby in a hospital where there is a neonatal unit.

Whether your baby will need to be looked after in the neonatal unit after they are born will depend on:

• their birthweight,
• whether they were born prematurely,
• whether they have any other health concerns.

You should have an opportunity to talk to one of the neonatal team before the birth of your baby if it is likely that they will need special care. You and your birth partner may also wish to visit the neonatal unit before your baby is born.

How your baby is born will depend on your individual circumstances and choices. If your baby is small but their umbilical artery Doppler measurements are normal on scan, you can choose to give birth vaginally. This often means your labour will need to be induced. Your baby’s heart rate will be monitored continuously during your labour. If the umbilical artery Doppler measurements are abnormal, you may be advised to have a caesarean birth.

You should attend hospital straight away if:

• you go into labour
• your waters (the sac of fluid that surrounds your baby) break, or
• your baby’s movements have reduced or changed.

If you have had a small baby before you are more likely to have a small baby again. After your baby has been born you should be advised whether there is anything you can do to reduce the chance of having a small baby in your next pregnancy.

Being told that you are having a small baby can be very stressful. If you are feeling anxious or worried in any way, please speak to your healthcare team who can answer your questions and help you get support. The support may come from healthcare professionals, voluntary organisations or other services.

Further information and resources are available on the NHS website.