Having a large baby
When you have a baby, one of the first things people like to know — besides the baby's name — is their birth weight. Even before the baby's born, parents can start to worry about its growth and development in the womb, particularly if they've been told they're having a big baby. But does size really matter?
Babies come in all shapes and sizes
More than 9 out of 10 babies born at term (37 to 40 weeks) weigh between 2.5kg and 4.5kg.
If your baby weighs 4.5kg or more at birth, they are considered larger than normal. This is also known as 'fetal macrosomia' and large for gestational age (LGA). (If they weigh less than 2.5kg, they may be considered smaller than normal.)
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.
Your doctor may also check the level of amniotic fluid. Excessive amniotic fluid, which surrounds the baby in the womb, can indicate the baby is larger than average, since larger babies can produce more urine.
However, there's no way of reliably measuring your baby's weight until after they are born. In many cases, women who are told they're going to have a large baby actually give birth to a baby within the normal range.
Why is my baby big?
A baby may be large at birth due to genetic factors, the mother's health or, in rare cases, a medical condition that causes the fetus to grow too quickly.
Several factors can contribute to large birth weight. For example:
- the baby's parents' height and stature
- if the baby is a boy (baby boys tend to be larger than baby girls)
- having older siblings (the chance of fetal macrosomia increases with each pregnancy)
- a previous pregnancy in which the baby was large
- being overdue by more than 2 weeks
- if the mother has diabetes during pregnancy
- if the mother gains a lot of weight during pregnancy or is obese
- if the mother is aged 30 or older
In some cases, larger-than-normal birth weight doesn't have a clear cause and can't be explained.
Giving birth to a large baby
Most large babies who weigh more than 4.5kg do not have a difficult birth. But there are still some risks associated with having a big baby.
Labour may take longer and be more likely to involve complications. There's an increased risk of having a forceps or vacuum-assisted delivery or a caesarean, and of birth injury to the mother or baby.
There is a link between fetal macrosomia and shoulder dystocia. Shoulder dystocia occurs during a vaginal birth when the baby's head has been born, but one of the shoulders becomes stuck behind the mother's pelvic bone. The baby's shoulders need to be released quickly so the baby's body can also be born, and they can start breathing.
Shoulder dystocia can happen during any birth. At least half of all babies who experience shoulder dystocia at birth weigh less than 4kg.
Large babies can be born via a normal, vaginal delivery — but it's best to give birth where you can access specialist medical services, just in case things don't go according to plan. Every pregnancy and 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
Since many large babies are born to mothers with diabetes, some babies will need help regulating their blood sugar after they're born. Later in life, the risk of childhood obesity or being overweight may increase and the baby may develop other problems with their metabolism.
A larger than average baby may need help breathing following the birth, so they may be admitted to the neonatal intensive care unit (NICU) or special care nursery (SCN).
There is also an increased risk of jaundice (yellowing of the skin) among large babies.
Regardless of their size at birth, 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 daily can also indicate that your baby is doing well.
Can you avoid having a large baby?
Often there's nothing you can do to avoid having a large or small baby. But looking after yourself during pregnancy is important for all women. You should consider:
- giving up smoking (if you currently smoke)
- eating a balanced, healthy diet
- maintaining your weight or, if overweight, losing weight before conception if possible
- if you have diabetes, trying to manage it properly
- avoiding alcohol and illegal drugs
Where to seek help
Always talk to your doctor, obstetrician or midwife first if you are concerned about your pregnancy, your own health or the health of your baby.
If you are worried about your baby's growth — or how fetal macrosomia might affect you as a mum — call Pregnancy, Birth and Baby on 1800 882 436 to speak to a maternal child health nurse.
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Last reviewed: September 2021