A small baby is a baby whose estimated weight puts them in the lowest 10% of babies at that stage of pregnancy. That means their weight is below 90% of other babies.
Your GP or midwife will monitor your baby’s growth throughout your pregnancy. You may be referred for tests if they are worried that your baby is:
- smaller than they should be at this stage in the pregnancy
- growing slowly
These tests may:
- measure the size of the baby and the blood flow through your placenta and umbilical cord. This is called a Doppler scan
- measure the fluid around your baby
- trace your baby’s heart in your 3rd trimester
Causes of a small baby
Your baby may be small for reasons that will not affect their health, for example if you or your partner:
- were small babies
- are Asian
Factors that increase the risk of a small baby
Your chances of having a small baby increase if you:
- have unregulated diabetes - talk to your GP about how pregnancy can affect your blood sugar levels
- take unprescribed drugs (especially cocaine)
- are underweight
- exercise excessively
You are also more likely to have a small baby if:
- you are over 40 when you become pregnant
- you have had a small baby before - tell your GP if you have previously had a baby who weighed less than 5lbs 8oz at birth
- there is a genetic condition causing your baby to be small
Restrictions to your baby’s growth
Your baby may be small because they are not growing as well as they should. This is called 'growth restricted'.
There are a number of reasons why your baby may be growth restricted, for example:
- your placenta isn’t working as it should - this may be due to high blood pressure or preeclampsia. Smoking and using drugs can also affect how healthy your placenta is
- an infection during your pregnancy has affected your baby
- your baby has another genetic or health problem causing them to be small and affecting their growth
If your baby has growth restriction, there is an increased risk of stillbirth.
Be aware of your baby's movements and contact your midwife or obstetrician immediately if there are any changes.
Read more about a change or reduction in your baby's movements
Your baby's delivery date will depend on their growth and scan measurements, if they are being monitored by your healthcare team.
If your baby is growing and the Doppler scan is normal, it is best to plan for delivery after 37 weeks. If there are no other complications, you may be able to have a vaginal birth.
You should have your baby in a maternity hospital which has a special care baby unit (SCBU) or a neonatal intensive care unit (NICU).
You may need to give birth early if your healthcare team feel that your baby would be put at risk by carrying them to full term. In this case, you may be offered a course of steroid injections during your pregnancy. This is to help your baby’s lungs after birth.