What causes babies to be small for gestational age?

Some babies may be smaller than others during pregnancy. However, what’s important is that your baby grows at a healthy rate in your womb, regardless of their size.

Here’s what you need to know about your pregnancy if you have been told by your doctor or midwife that your baby is small or growing slower than expected. Your doctor or midwife may refer to a slower growth as ‘small for dates’, ‘small for gestation’ or ‘fetal growth restriction’.

Why is my baby small?

Most babies who are smaller than expected are healthy. In some cases, babies are small and appropriately grown, given the size of their parents and their ethnicity.

However, up to 10% of pregnancies will be affected by ‘fetal growth restriction'. This is a more serious complication that means your pregnancy must be closely monitored. In many instances, babies with fetal growth restriction are small because the placenta hasn’t developed well enough to keep up with the baby’s growing needs for nutrients and oxygen, although a cause may not always be found.

Risk factors for having a baby with fetal growth restriction include:

How is a baby's size measured?

During routine antenatal check-ups, your doctor or midwife will use a tape measure to estimate the growth and size of your baby by measuring your abdomen, from your pubic bone to the top of your uterus (or fundus). This is known as the ‘symphyseal fundal height measurement’ (SFH) and gives some indication about a baby’s growth during pregnancy.

If you have risk factors for fetal growth restriction, or if your baby has been diagnosed with this, your baby’s growth may be monitored more closely by ultrasound, instead of using a tape measure.

Every pregnancy is unique. Your doctor or midwife will track your baby’s growth at each check-up. They will talk with you about next steps if there are signs that your baby’s growth has slowed.

Should I be concerned if my baby is small?

Most small babies will be healthy. However, if your baby has fetal growth restriction, this increases the risk of complications and, sadly, also the risk of stillbirth. Your doctor and midwife will monitor you and your baby’s health closely to reduce the risk of this happening.

You can monitor your baby’s health by attending your scheduled antenatal visits and keeping a close eye on your baby’s movements. If you are concerned about your baby’s movements at any stage, call your doctor or midwife immediately.

Your doctor or midwife may also talk to you about the best time for your baby to be born. Sometimes it may be necessary for your baby to be born early, before your due date. Every situation will be different; your doctor or midwife will give you personalised advice each step of the way.

Generally a baby with fetal growth restriction can still be born via normal vaginal delivery, but you and your baby may need extra care both during labour and after the birth. This means 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.

Can I do anything to reduce the risk of having a small baby?

Often there is nothing you can do to prevent a baby being small. But looking after yourself during pregnancy is important to help reduce the risk of having a baby with fetal growth restriction. You should:

What to expect if your baby is born with low birth weight?

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 or those who are born prematurely, may need to be admitted to the neonatal intensive care unit (NICU) or special care nursery (SCN).

Some of the challenges faced by babies with low birth weight can include:

• breathing or heart problems
• low oxygen levels at birth
• difficulty maintaining 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 conditions are more likely to occur in premature babies and those with fetal growth restriction.

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.

Here are some questions you might want to ask your midwife or doctor:

• What does having a small baby mean for me and the health of my baby?
• Will I need more regular check-ups during pregnancy? If so, what does this include?
• How will I know if my baby is well?
• Will having a small baby affect how and where I can give birth?
• Does having a small baby affect my next pregnancy?

Who can I talk to for advice and support?

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

Sources:

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: April 2022

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Summary of vaccines reviewed and level of evidence concerning vaccine safety.