Social media is buzzing about baby boy Cassian, who has captured the internet’s attention for his size. Born earlier this year in the United States, Cassian weighed 5.8 kilograms at birth. After his mother and the hospital shared the news, headlines about the “giant” baby spread around the world. While his weight is above average, Cassian is not the only hefty newborn; a baby in Brazil in 2023 weighed 7.3 kilograms.

But what does it really mean to have a big baby, and should mums be worried about complications?

What counts as a big baby?

Babies are considered big when they weigh over 4 to 4.5 kilograms, a condition known as macrosomia. Another way to define it is if the baby is above the 90th percentile for weight at a certain gestational age. This means the baby weighs more than 90 per cent of babies at the same stage of pregnancy.

A more accurate term is “large for gestational age,” as it considers both the baby’s weight and weeks of gestation. Despite media coverage, the proportion of large babies in Australia has stayed around 9–10 percent over the last decade.

Are there risks for big babies and their mothers?

We don’t know the specific circumstances of Cassian’s birth or the health of him or his mother. However, generally speaking, bigger babies, especially over 4.5 kilograms, can increase the likelihood of complications.

For mothers, there may be a higher need for interventions such as forceps, vacuum delivery, or a caesarean section. These procedures can affect recovery and future birth options.

For babies, shoulder dystocia, where the shoulders get stuck during birth, is a risk.

Obstetricians and midwives may need to perform extra manoeuvres to deliver the baby safely, which can occasionally cause injury or oxygen restriction. Thankfully, such complications are rare and often occur when a large baby is not expected.

What causes babies to be big?

Most big babies are healthy, and there are several reasons why they may grow larger:

  • Genetics play a role. Some families simply have bigger babies. Babies who go past their due date tend to be bigger.
  • Diabetes, especially if poorly controlled, can lead to bigger babies because excess sugar is stored as fat in the baby. Gestational diabetes can also increase the child’s risk of obesity and diabetes later in life.
  • Mothers who are larger before or during pregnancy may have bigger babies due to a higher chance of diabetes or nutritional factors.

Can you predict a big baby?

Predicting a baby’s birth weight is tricky. Many women are told they may have a big baby but give birth to an average-sized child, and vice versa.

Midwives and obstetricians measure the uterus during antenatal check-ups, considering the baby’s position and the height of the uterus relative to the belly button.

They also measure from the top of the uterus to the pubic bone. For example, at 36 weeks, this measurement is usually 34–38cm. A difference of more than 3cm often leads to an ultrasound to check the baby’s growth.

Even ultrasounds are not very accurate. The Big Baby Trial in the UK showed that 60 percent of babies predicted to be big were not actually large at birth.

The study also found little difference in complications, like shoulder dystocia, whether labour was induced early or allowed to start naturally.

What can women do to stay healthy?

  • The best advice is to aim for a healthy weight before pregnancy, ideally with a BMI under 30.
  • Eating a balanced diet, limiting foods high in saturated fats and sugar, avoiding excessive weight gain during pregnancy, and exercising regularly all help.
  • Women with diabetes or gestational diabetes should monitor blood sugar and the baby’s growth closely.
  • Regular check-ups with a midwife or obstetrician provide support and guidance to ensure both mum and baby stay healthy.