Why is my baby large?

When you have a baby, one of the first things people like to know is their birth weight. Birthweight is important as it's an indicator of your baby's health.

There is not one definition for a large baby. Generally, if your baby weighs over 4.5kg at birth, they are considered large. This is also known as 'fetal macrosomia' and 'large for gestational age (LGA)'.

If your baby weighs less than 2.5kg at birth when at term (after 40 weeks of pregnancy), they may be considered smaller than normal.

In 2021, just over 1 in 100 babies who were born in Australia weighed over 4.5kg.

However, more than 9 out of 10 babies born at term (37 to 40 weeks) weigh between 2.5kg and 4.5kg.

How is a baby's size measured?

There's no way of reliably measuring your baby's weight until after they are born. In many cases, people who are told they're going to have a large baby actually give birth to a baby within the normal weight range.

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

An ultrasound scan can also give your doctor or midwife an idea of how big your baby is likely to be. However, it's often not very accurate.

Should I be concerned if my baby is big?

A baby may be large at birth due to both genetic and environmental factors.

Factors relating to you include:

• pre-existing diabetes
• gestational diabetes
• your pre-pregnancy body mass index (BMI)
• if you gain a lot of weight during pregnancy
• your race
• if you are aged 30 years or older
• having already had a large baby
• being overdue by more than 2 weeks

Factors relating to your baby are the sex of your baby — baby boys tend to be larger than baby girls.

In some cases, the size of your baby doesn't have a clear cause and can't be explained.

Questions you might want to ask your midwife or doctor

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

• What does having a large 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 large baby affect how and where I can give birth?
• Will having a large baby affect my next pregnancy?

What should I expect when I give birth to a large baby?

Most large babies do not have a difficult birth. But there are still some risks associated with having a big baby.

Large babies can be born via a normal, vaginal delivery. You may choose to give birth where you can access specialist medical services, just in case things don't go according to plan.

Labour may take longer and be more likely to involve complications. There's an increased risk of having a caesarean birth, and of birth injury to you and your baby.

Elective caesarean births may be recommended for:

• women without diabetes who are suspected of having a baby that weighs 5kg or more
• women with diabetes who are suspected of having a baby that weighs 4.5kg or more

Your doctor or midwife will explain the risks and benefits of vaginal birth and caesarean section based on your circumstances.

Perineal tears

Your risk of perineal tears may increase with a large baby. This is especially true when should dystocia occurs.

Shoulder dystocia

There is a link between having a large baby and shoulder dystocia.

Shoulder dystocia can occur during a vaginal birth. It's when the baby's head has been born, but one of their shoulders becomes stuck behind your pelvic bone. Your baby's shoulders need to be released quickly so that their body can also be born. This also lets them start breathing.

Shoulder dystocia can result in fracture of the clavicle (collar bone). Sometimes the nerves in the shoulder can become damaged. However, this resolves in over 8 out of 10 babies by the age of 1 year.

Shoulder dystocia can happen during any birth. At least half of all babies who experience shoulder dystocia at birth weigh less than 4kg.

Every birth is unique, so talk to your doctor or midwife about the best place for you to give birth.

After the birth of a large baby

Regardless of their size, a baby's weight is always monitored closely after they are born. This is to check that they are healthy and growing properly.

Large babies are more likely to have a low Apgar score at 5 minutes.

If your baby needs help breathing following the birth, they may be admitted to: the neonatal intensive care unit (NICU) or special care nursery (SCN).

If you have diabetes, your baby may need help regulating their blood sugar after they're born.

There is an increased risk of jaundice (yellowing of the skin) among large babies.

When they are older, your child may be at increased risk of being overweight or obese and developing metabolic syndrome.

How do I know if my baby is doing well?

Your baby's weight isn't the only thing that's important. Other things that show that your baby is doing well are:

• how well they are feeding
• the number of wet nappies and poos they have daily

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

Often there's nothing you can do to change the size of your baby. But looking after yourself during pregnancy is important. You should consider:

• eating a balanced, healthy diet — especially if you have gestational diabetes
• exercising during pregnancy
• if overweight, losing weight before conception, if possible

A low glycaemic diet has been found to be very effective in reducing the risk of having a large baby for pregnant women with gestational diabetes.

General tips for a healthy pregnancy are:

• giving up smoking — if you currently smoke
• avoiding alcohol and illicit drugs

Resources and support

Always talk to your doctor or midwife if you are concerned about your health or the health of your baby.

If you are worried about your baby's size — or how fetal macrosomia might affect you — call Pregnancy, Birth and Baby to speak to a maternal child health nurse.