Baby boy Cassian is an internet sensation. He was born earlier this year in the United States weighing 5.8 kilograms. But after his mum and the hospital shared the news recently, it wasn’t long before headlines about the “giant” baby spread around the world. These included:

‘Are you OK’?: Woman breaks record with giant newborn baby

Record-breaking baby tips the scales at almost double the average size of a newborn

While baby Cassian was born heavier than average, he’s not unique. There have been other examples in the news of babies born heavier. That includes a baby boy born in Brazil in 2023 who weighed 7.3kg.

These stories might make women all over the world cross their legs. But how common are big babies, and does their birth always lead to complications?

What are big babies?

Macrosomia describes babies born over 4kg or 4.5kg, depending on the definition.

A big baby can also be defined as having a birth weight over the 90th percentile at a particular gestational age. In other words, more than 90% of babies have a lower birth weight at this particular stage of the pregnancy. The term “large for gestational age” is probably a more accurate term as the weeks of gestation is used alongside the weight.

There has been little change overall in the percentage of large babies in the past decade in Australia. While stories of such births hit the media, their proportion hovers around 9–10% of births.

What are the problems for big babies and their mums?

We don’t know the specific circumstances of Cassian’s birth, his health or that of his mother. And we don’t know whether common reasons for larger babies are relevant in this situation.

But, generally speaking, birth complications can be higher for mothers and babies when the baby is big, especially if more than 4.5kg. This is certainly not always the case, however.

There is an increased need for interventions during the birth, such as forceps or vacuum delivery, or a caesarean section the bigger the baby is. Having these interventions can impact a women’s recovery after the birth, and options for the next birth.

For the baby there are higher risks of the shoulders getting stuck in the birth canal during the birth (known as shoulder dystocia).

Midwives and obstetricians also may need to make extra manoeuvres for the baby to be safely delivered. For instance, they may need to try and bring down one shoulder if it’s stuck behind the mother’s pubic bone.

These manoeuvres can damage the baby or lead to oxygen restrictions, with the baby needing to be resuscitated. However, these complications are rare and can occur when a big baby was not expected.

What leads to a big baby?

Big babies are most often healthy babies, and there are a number of reasons for them.

Genetic factors mean babies are always big in some families.

Babies that go over their due dates tend to be a bit bigger as they have more time to grow inside their mothers.

Having diabetes, especially if this is poorly controlled, can lead to larger babies. This is because the mother’s higher blood sugar leads to the baby receiving more energy than it needs, so it stores this extra energy as fat.

Babies of mothers with diabetes diagnosed for the first time in pregnancy (gestational diabetes) are at increased risk of being obese and developing diabetes in the future.

Mothers who are larger before pregnancy, or when pregnant, may also be more likely to have big babies. This is mostly due to the increased likelihood of developing diabetes in pregnancy, and perhaps poorer nutrition choices.

Can you predict a big baby?

Estimations of babies’ weights before they are born are imprecise. That’s why so many women are told they are going to have a big baby and don’t, and others are surprised by a big baby when it arrives.

Midwives and obstetricians routinely feel a woman’s growing uterus when they provide antenatal check-ups. They are looking at the position the baby is lying in the uterus as well as where the top of the uterus is compared to the woman’s belly button. This gives an idea of whether the baby is growing as you would expect at that time.

They also measure from the top of a woman’s belly to the top of her pubic bone with a tape measure. The weeks of pregnancy usually correspond to the measurement within a couple of centimetres.

For example, at 36 weeks of pregnancy the tape measurement would be somewhere between 34cm and 38cm. If there is more or less than a 3cm difference between the measurement and the numbers of weeks of pregnancy then an ultrasound would be offered to look at how the baby’s growing and to estimate the size.

But ultrasounds are poor predictors of actual birth weight. The Big Baby Trial was published earlier this year. It randomised nearly 3,000 women in the United Kingdom to being induced at 39 weeks if suspected to be having a big baby (according to an ultrasound) or waiting for labour to start.

There was little difference in birth weight or poor outcomes, such as shoulder dystocia for the baby, leading to the trial being stopped early. Around 60% of babies screened as being big babies were not actually big at birth, showing the inaccuracy of ultrasounds in predicting birth weight.

What can women do?

The best health advice for women is to try to be a healthy weight (under a BMI of 30) before getting pregnant.

Eat a balanced diet and limit your intake of foods and drinks high in saturated fats and sugar. Try not to put too much weight on during pregnancy and exercise regularly. Talk to your midwife or obstetrician for advice and support about this.

If you have diabetes, or if this has been diagnosed during the pregnancy, close monitoring of your blood sugar and baby’s growth is important.

Hannah Dahlen, Professor of Midwifery, Associate Dean Research and HDR, Midwifery Discipline Leader, Western Sydney University

This article is republished from The Conversation under a Creative Commons license. Read the original article.