What Is Considered To Be A Big Baby?

What Is Considered To Be A Big Baby?

At some stage during pregnancy, it’s pretty safe to say that all women will wonder how big their baby will be at birth.

The average birth weight of a newborn is 3.4kg (7lb 8oz), although there is a wide range of what is considered a normal weight.

Remember, it takes smaller babies and bigger babies to calculate the size of an ‘average’ baby, and nearly all healthy, full term babies are within the range of normal.

However, a significant amount of women are being told their baby is too big to be born naturally, only to find after the birth that their baby was not in the range of what is considered to be a big baby.

They were more likely to have interventions, including c-section, which increases complication risks unnecessarily.

What Is The Definition Of A Big Baby?

The technical term for a big baby is macrosomia. There seems to be different opinions on what actually defines fetal macrosomia. Some experts believe it is a birthweight of more than 4 kilograms (8lb 13oz) and other experts define macrosomia as being greater than 4.5 kilograms (9lb 15oz).

Fewer than 2% of babies in the US and Australia are born over 4.5 kilograms, with less than 9% in the US born over 4kg.

Suspected big babies are one the major reasons why women are scheduled inductions or elective c-sections. Growth and development checks during pregnancy can guess the general size of your baby, but there is no accurate way to measure your baby until it is born.

Are We Getting It Wrong?

Recent research suggests women are being told their babies are too big incorrectly. The Listening to Mothers III survey reported over 31% of mothers were told by their care provider their baby was large, yet less than 10% of those women had babies weighing more than 4kg. The remaining women had babies weighing less than 3.9kg (8lb 13oz).

Women with suspected large babies were almost twice as likely to have medical interventions, such as having labour medically induced or attempting to bring on labour themselves. They were also were more likely to ask for induction and have c-sections than other women.

Another study found women with suspected big babies had higher rates of complications when compared to women who had unexpected big babies.

  • Induction rates for unexpected big babies was 14% compared to 42% for suspected big babies.
  • C-section rate for unexpected big babies was 17% compared to 52% for suspected big babies
  • Complication rate for women with unexpected big baby was 4% compared to 17% with suspected big baby.

This research suggests the way we manage labour for large babies is not improving outcomes for either women or their babies.

The American College of Obstetricians and Gynecologists (ACOG) states in their 2014 guidelines that c-section should be used rarely, and only when babies are estimated to weigh at least 5 kilograms in non diabetic women and over 4.5 kilograms in diabetic women.

The guidelines also note ultrasound scans in late pregnancy are associated with a higher likelihood of a c-section with no evidence of improved outcome for babies.

Does Size Really Matter?

What size your baby will be at birth is influenced by many factors.

Big babies can run in families, and be due to social factors as well as genetics. If your parents are overweight, you are more likely to be overweight, and will be more likely to have a bigger baby. Likewise, if you have a family history of diabetes, you may increase the chance of having a larger baby. Diet and lifestyle issues are a huge contributor to gestational diabetes, with research finding a healthy lifestyle (no smoking, exercise, nutritious diet) reduced gestational diabetes by up to 83%. Find out more here.

However, it’s important to understand that a vast majority of women who have a ‘large’ baby will have a normal vaginal birth with no complications. Having a large baby doesn’t mean labour will necessarily be more difficult, despite the common perception that it is harder work. On the other hand, unnecessary interventions can make labour more difficult for both mother and baby.

No matter what size your baby is, factors that determine how difficult your birth is can usually be resolved. This includes choosing care providers who support natural birth and being undisturbed during labour, to be upright and active as you wish. The position of your baby also influences labour progression, regardless of size.

We’re often told to imagine something the size of a watermelon trying to exit through an opening the size of a lemon. Babies’ heads are the largest part of their body, and are designed to mould to fit through the cervical opening. Women’s bodies are designed to loosen during pregnancy and open during labour – hormones flooding their bodies ensure that happens (the pregnancy waddle is a sure sign of that!). Evolution has perfected the birth process so in the rare situations when a baby is large, we have ways of managing the process to allow normal birth to occur.

Having a suspected big baby may depend on your personal risk factors, such as genetics, general health and activity levels. Keep in mind big babies can’t be accurately diagnosed before birth, and choose a care provider who doesn’t routinely use growth scans, as these can cause unnecessary concern and medical procedures you may want to avoid.

Last Updated: April 5, 2016


Sam McCulloch enjoys talking so much about birth that she decided to become a birth educator and doula, supporting parents in making informed choices about their birth experience. In her spare time she watches Downton Abbey and has numerous creative projects on the go. She is mother to three beautiful little humans.

One comment

  1. All my babies have been larger than average. The smallest was 8 lbs, 7 ounces, the largest I had just last month at 11 lbs, 4 ounces. No, we didn’t know he would be that large because i declined a late sonogram. I delivered him, and the others, vaginally and without any drugs. I’d say the labor wasn’t too bad, either. I delivered in a hospital, with a midwife. She was awesome. She let me move how I felt I needed to move, respected my wish to be off the fetal monitor, and we had an amazing experience.

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