Having a small baby

  • An unborn baby is small if, at that stage of pregnancy, his or her size or estimated weight on scan is in the lowest 5% of babies. This means the smallest five out of every 100 babies.

  • Your baby’s weight is affected by many things, including:
    • Your height and weight – taller, heavier women tend to have heavier babies
    • Whether you or your partner were a small baby
    • Your ethnicity – for example, South Asian women tend to have smaller babies
    • The number of babies you have had – babies tend to become heavier with each pregnancy
    • Whether your baby is a boy or a girl – boys tend to be heavier.

  • Your baby could be small because of a combination of the factors above. If this is the case, your baby is likely to be healthy because he or she is meant to be small.
  • However, sometimes babies are small because they do not grow as well as expected. This is called being ‘growth restricted’. Causes of growth restriction include:
    • The placenta not working as well as it should – this could be because of medical problems such as high blood pressure or complications of pregnancy such as pre-eclampsia (you can find out more about pre-eclampsia from the patient information Pre-eclampsia smoking, using drugs or being very anaemic can also affect how your placenta works
    • An infection during pregnancy that affects the baby
    • Having a baby with a developmental or genetic problem.

  • Lifestyle choices such as smoking, using cocaine, high amount of alcohol or not eating healthily are all linked to an increased chance of the baby being growth restricted.
  • You are more likely to have a baby that is growth restricted if you are over 40 or have high blood pressure, kidney problems or diabetes complications. Having lost a baby late in pregnancy or having had a small baby in the past also increases your risk.
  • Heavy vaginal bleeding, especially in the second half of pregnancy, can also affect the way your baby grows.

  • Some of these risks cannot be changed, but some can:
    • Reduce or quit smoking – your midwife or GP can refer you for support to help you stop smoking.
    • Do not use recreational drugs, especially not cocaine.
    • Leading a healthy lifestyle and eating healthily reduces the risk of having a small baby. If you are at increased risk of pre-eclampsia, you may be advised to take low-dose aspirin (75 mg once a day) from 12 weeks of pregnancy until your baby is born.

  • If your baby is small but healthy, he or she is not at increased risk of complications.
  • If your baby is growth restricted, there is an increased risk of stillbirth (the baby dying in the womb), serious illness and dying shortly after birth. The earlier in pregnancy and the more severely your baby’s growth is affected, the more likely it is that your baby will have a poor outcome. Babies whose growth is only affected later in pregnancy have a better outcome.
  • Most babies affected by infection or by developmental or genetic problems have severe growth restriction and are usually detected early.
  • Once your doctor has identified that your baby is small, you will be offered extra monitoring to keep an eye on your baby’s growth and wellbeing. You are likely to be advised to have your baby early to be as certain as possible that your baby will be born healthy.

  • Your midwife or obstetrician should assess your risk of having a small baby in early pregnancy:

    • If you are at low risk of having a small baby, your midwife or obstetrician will still monitor your baby’s growth: At each antenatal appointment, from 24 weeks of pregnancy onwards, the distance between your pubic bone and the top of your womb (symphysis fundal height) should be measured and plotted on a chart. Recording this measurement should give reassurance that your baby is growing normally.
    • If the growth slows down or the measurement suggests that your baby may be small, you will be advised to have an ultrasound scan.

  • If you are at increased risk of having a small baby, you may be referred for:

    • Regular ultrasound scans from 26–28 weeks of pregnancy onwards
    • An ultrasound scan of the blood flow to your placenta – this is known as the uterine artery Doppler test and is done at 20–24 weeks of pregnancy; depending on the results, you will be advised whether or not your baby needs a further scan.

  • You may have the following tests to check your baby’s wellbeing:
    • Umbilical artery Doppler – this measures the flow of blood through the umbilical cord
    • MCA Doppler - this measures the flow of blood through the umbilical cord
    • Measuring the amount of amniotic fluid around your baby.
  • You may be referred to a fetal medicine specialist for more frequent and detailed scans if the umbilical artery Doppler test is abnormal.

  • This will depend on how affected your baby’s growth and on the Doppler measurements. The scans will help your team decide whether it is better for your baby to be born early or to continue your pregnancy longer. If your baby is growing and the Doppler tests are normal, it is usually best to wait until you are at least 37 weeks pregnant.

  • Depending on the timing of birth, you may be offered a course of corticosteroids for lung maternity over a 24–48 hour period. This is to help your baby’s development and reduce the chance of breathing problems after birth.

  • If there are no other complications, you may be able to have a vaginal birth. Your baby will be monitored closely during labour. However, if the Doppler measurements are abnormal, your doctor may recommend that your baby be born by caesarean section.
  • If you go into labour, if your waters have broken or if you have had any bleeding before the date you should attend hospital straight away.

  • Whether your baby will need to be looked after in the neonatal unit will depend on how small your baby (weight of baby) is and at what weeks of pregnancy your baby is born. Baby’s lung capacity (Neonate intensive care units).
  • This information is not for women who are pregnant with more than one baby.

  • This information aims to helps you make the best decision regarding your care. It is not meant to replace advice from a doctor about your own situation.