Why is my baby small?

The most common reason why a baby is smaller than average — weighing less than 2.5kg at birth — is prematurity (being born before 37 weeks’ gestation). The earlier the baby is born, the smaller they are likely to be.

This is because the baby will have had less time in the womb to grow. A baby gains much of its weight in the last weeks of the pregnancy.

Some babies don’t gain much weight in the womb because of other risk factors. Having one or more of these risk factors does not mean you will have a baby with low birth weight, but may be at higher risk of having a baby with low birth weight. Risk factors include:

• having a multiple pregnancy (twins, triplets or more)
• a problem with the placenta
• an infection in the womb
• the mother having a chronic health condition, such as high blood pressure or kidney disease
• the mother taking certain medicines to treat conditions such as epilepsy and blood clots
• the mother not gaining enough weight or not maintaining her weight throughout the pregnancy
• a previous pregnancy that resulted in the baby having a low birth weight
• smoking, drinking alcohol or using illegal drugs
• exposure to air pollution or lead
• the baby having a congenital disorder

Sometimes a baby is born weighing less than 2.5kg simply because the parents are small themselves or have a certain ethnic background. For example, babies born to Indigenous mothers in Australia are twice as likely to have low birth weight than those born to non-Indigenous mothers.

How is a baby's size measured?

During routine antenatal check-ups, your doctor or midwife may estimate the growth and size of your baby by measuring the fundal height. That is the measurement from your pubic bone to the top of your uterus.

An ultrasound can also give health professionals an idea of how big your baby is likely to be — but it's not very accurate.

There's no way of accurately measuring your baby's weight until after they are born.

A baby's weight is always monitored closely after they are born to make sure they are healthy and growing properly. But their weight isn’t the only thing that’s important. How well they are feeding and the number of wet nappies and poos they produce can indicate whether or not your baby is doing well.

Should I be concerned if my baby is low weight?

Small and large babies can both be born via normal, vaginal delivery, but you and the baby will probably need some extra care both during labour and after the birth. So it’s best to give birth where you can access specialist medical services. Talk to your doctor or midwife about the best place for you to give birth.

If your baby weighs less than 2.5kg at birth, their head may appear to be a lot bigger than the rest of their body. They may look thin with little body fat.

Babies of low birth weight may need to be admitted to the neonatal intensive care unit (NICU) or special care nursery (SCN). Some of the challenges sometimes faced by small babies include:

• breathing or heart problems
• low oxygen levels at birth
• an inability to maintain their body temperature
• difficulty feeding and gaining weight
• infection
• bleeding on the brain (called ‘intraventricular haemorrhage’)
• problems with their eyes and vision
• problems with their intestines

These are all more likely to occur in premature babies.

If there are no other complications, low birth weight babies usually 'catch up' in their physical growth. In later life, however, people who were born smaller than average are more likely to develop diabetes, obesity, heart problems and high blood pressure.

Can I prevent low birth weight?

Often there’s nothing you can do to prevent a baby being born small — or large. But looking after yourself during pregnancy is important for all women. You should consider:

• quit smoking (if you currently smoke)
• eating a balanced, healthy diet
• maintaining a healthy weight during pregnancy
• avoiding alcohol and illegal drugs

Where can I find help?

Always talk to your doctor, obstetrician or midwife first if you have any concerns about your pregnancy, your own health, or the health of your baby.

You can also can call Pregnancy, Birth and Baby on 1800 882 436 to speak to a maternal child health nurse.