Big baby (macrosomia): what it means, how accurate scans are and your birth options

Macrosomia is the medical word for a big baby. KKH describes large babies as those weighing over 4kg, and NUH says 3% of full-term babies here are born heavier than that. If your gynae says your baby is measuring big, ask how the weight was estimated, keep your blood sugar and weight gain on track, and plan the birth together. NUH notes that a scan estimate can be 10 to 15% too high or too low.

What to do if your baby is measuring big, step by step

1. Ask how the estimate was made, and what weight it gives. NUH says your gynae first measures your bump's fundal height, then may order an ultrasound if it seems large for the week.
2. Check that you've had the gestational diabetes test. KKH says all pregnant women in Singapore are offered an oral glucose tolerance test (OGTT) between 24 and 28 weeks, and our gestational diabetes guide explains your results.
3. Follow your team's blood sugar targets if you have gestational diabetes, and use our gestational diabetes diet guide for meals. NUH says you may need medicine, usually insulin, if diet and exercise aren't enough or a scan shows a larger baby.
4. Keep your weight gain in range with the pregnancy weight gain calculator. NUH puts a typical healthy gain in a first pregnancy at 10 to 16kg, and says US studies link gaining over 18kg with double the risk of a big baby.
5. Stay active if your gynae is happy for you to. KKH suggests at least 30 minutes of exercise every day to lower the risk of gestational diabetes and gaining too much weight.
6. Ask about birth options well before labour starts. NUH says the safest way to deliver a big baby is decided between you and your doctor, and with gestational diabetes KKH also weighs your blood sugar control and the baby's growth and size.
7. Keep track of your baby's movements each day with the baby kick counter. If your baby moves less than usual, KKH says to come in to its Delivery Suite from 22 weeks rather than wait for a check-up.
8. Plan to hold and feed your baby soon after birth. HealthHub encourages skin-to-skin contact and early feeding if you have gestational diabetes, because your baby has a higher chance of low blood sugar.

What counts as a big baby

Doctors measure big in two ways. The first is a fixed birth weight: KKH lists a past baby over 4kg as a risk factor for gestational diabetes, and HealthHub uses the same 4kg mark. The second compares your baby with others at the same week of pregnancy, which is called large for gestational age. The UK's NICE guideline treats a baby as suspected large when the estimated weight is above the 95th percentile at or after 36 weeks.

For scale, NUH says the average full-term Chinese baby in Singapore weighs about 3.2kg.

Why a scan can be off

At antenatal visits your gynae feels your tummy and measures the fundal height, the distance from the top of your womb to your pubic bone, which NUH says roughly matches your weeks of pregnancy. If your bump seems large for the week, an ultrasound comes next. It works out weight from the size of the baby's abdomen, and NUH gives it an error margin of 10 to 15% either way. Treat the number on the report as a best guess. If it's driving a big decision, ask what else your gynae is weighing it against.

What makes a baby grow big

NUH lists genes (both parents' height and race), diabetes, a past big baby, obesity before pregnancy, heavy weight gain and a pregnancy that runs past the due date. It also says most big babies have no clear cause, and no single factor predicts a heavy birth weight.

Diabetes is the cause you can do the most about. KKH says that when gestational diabetes goes undetected or untreated, the baby may grow larger than normal. If you had diabetes before pregnancy, our high risk pregnancy guide covers how your care changes. Near your due date? The 40 weeks pregnant guide explains what happens if you go past it.

Risks for you and your baby

Most of the concern is about the birth. In shoulder dystocia, the baby's head is out but one shoulder gets stuck above your pubic bone. KKH says this happens in about one in 150 vaginal births. It's more likely with large babies, diabetes, a BMI of 30 or more, or an induced labour. KKH adds that in most cases it can't be predicted. NUH cites a local study where babies over 3.6kg had a 16-fold higher risk than lighter babies.

NUH says the baby's shoulders are freed with special manoeuvres. KKH says a nerve injury in the neck can stop the arm moving well, but this is usually temporary and permanent damage is rare. A broken arm or shoulder bone can happen, and most heal very well. KKH also lists brain damage from a lack of oxygen as a possible complication.

For you, HealthHub says a bigger baby can make the delivery harder. NUH links high blood sugar with a higher chance of induced labour or a caesarean. After birth, HealthHub says poorly controlled gestational diabetes raises the chance that your baby needs the neonatal unit for breathing problems, low blood sugar, mineral imbalances or jaundice.

What your gynae may discuss

Your gynae may raise some of these, depending on your scans, your blood sugar and your past births.

NUH says a caesarean carries risks of infection, bleeding that needs a blood transfusion and blood clots in the legs, plus higher risks in later births. Our natural birth guide covers labour, and our C-section guide covers the operation and recovery.

If you don't have diabetes, the UK's NICE guideline gives three options: waiting for labour, induction or a caesarean. NICE says induction lowers the risk of shoulder dystocia but raises the risk of a severe tear. It found the same risk of the baby dying, a nerve injury to the arm or an emergency caesarean either way. NICE asks doctors to respect the woman's choice.

Your baby's blood sugar checks after birth

If you have gestational diabetes, your baby's blood sugar can drop after birth. NUH says your baby's blood glucose will be tested, and a low reading is treated until it's back in the normal range. Some babies need extra care in the neonatal unit, NUH adds. During labour, NUH's team also watches your own blood sugar closely.

Feeding early helps. HealthHub encourages skin-to-skin contact and feeding as soon as possible after delivery. KKH supports skin-to-skin and starting breastfeeding within the first hour of birth, and it encourages breastfeeding after gestational diabetes. If your baby is big but you don't have diabetes, ask whether your hospital checks the baby's sugar too.

When to see a doctor

• Your baby is moving less than usual. KKH lists this as a medical concern to come in for, so don't wait for your next check-up. The kick counter helps you notice a change.
• Strong, regular contractions, a blood-stained show, or a gush or steady trickle of clear fluid. KKH lists these as signs of labour. Our stages of labour guide explains what comes next.
• Vaginal bleeding, fever, a severe headache, changes in your vision or pain in your tummy. KKH lists each of these as a reason to come in.
• Glucose readings that keep going above your targets. HealthHub says your doctor may add medicine when diet and exercise no longer control your blood sugar, so call your clinic.
• Strong thirst, passing urine much more often, or weight loss. KKH says these can happen when blood sugar is very high.

At KKH, go to the Delivery Suite on Level 2 of the Women's Tower if you're 22 weeks or more. Before 22 weeks, go to the Urgent O&G Centre in Basement 1. If you deliver elsewhere, call your gynae's clinic or your hospital's delivery suite. For an emergency, KKH says to call 995 or go to an emergency room. Our premature labour guide has more on where to go.

Frequently asked questions

Is a big baby pregnancy (macrosomia) dangerous?

The main concern is shoulder dystocia at birth. KKH puts it at about one in 150 vaginal births, with large babies more at risk, and says most nerve injuries from it are temporary. HealthHub says that with good sugar control, most mothers with gestational diabetes have smooth deliveries and healthy babies.

Can I have a vaginal delivery with a big baby?

NUH says vaginal birth is generally encouraged for all women unless there's a medical reason for a caesarean. For a big baby, NUH says the safest way to deliver is decided between you and your doctor. Ask how your hospital's team handles shoulder dystocia.

My baby is measuring big on the scan. Is it accurate?

It's an estimate. NUH says scan weights have an error margin of 10 to 15%, too high or too low. Your gynae also looks at your bump measurement and, if you have gestational diabetes, your blood sugar control.

What does large for gestational age mean?

It means your baby is bigger than expected for the week of pregnancy. It isn't tied to one weight. NICE uses an estimated weight above the 95th percentile at or after 36 weeks.

How common is macrosomia in Singapore?

NUH's figures for full-term babies here: about 10% weigh more than 3.6kg, and 3% weigh more than 4kg. Gestational diabetes, the main cause you can change, affects about one in five pregnant women in Singapore, NUH says, and KKH says every pregnant woman here is offered the OGTT.