In some parts of England babies are up to four times more likely than others to be larger than usual.
A range of factors can influence the likelihood of having an unusually large baby, including a mother’s weight and age, genetics, gestational diabetes, and a baby being overdue.
In Crawley, 11.6% of babies weighed over four kilograms. A total of 1,370 babies were born in Crawley, with 140 weighing between 4,000g and 4,999g and just five weighing over five kilograms.
In neighbouring Horsham, 14.4% of babies weighed over four kilograms. Of the 1,235 babies born in Horsham in 2021, 105 weighed between 4,000g and 4,999g while ten weighed over five kilograms.
In Mid Sussex, 12.2% weighed over four kilograms. Of the 1,250 babies born in the area, 135 weighed between 4,000g and 4,999g. Just five babies in Mid Sussex tipped the scales at over five kilograms.
How big is a big baby?
Babies are considered unusually large if they weigh at least 8lb 13oz, or four kilograms – the equivalent of four bags of sugar. The medical term is foetal macrosomia.
Babies this size can lead to complications during labour, and mothers are more likely to need a caesarian section.
NationalWorld’s analysis of data from health bodies across the UK nations shows between one in seven and one in ten babies have macrosomia, with those in Northern Ireland most likely to be on the chunkier side.
The latest data for each nation covers different time periods so the comparison between countries is imperfect. There are also differences in what the data covers, with some countries excluding stillbirths or multi-baby births.
But within each nation there is enormous regional variation in the proportion of babies born with macrosomia.
We explore the figures for each nation in more detail below – and reveal the local areas with the biggest babies.
Statistics in England
Office for National Statistics (ONS) data shows one in ten babies in England are unusually large.
Of 569,314 live births with weights recorded in 2020, 57,753 babies (10.1%) tipped the scales at four kilograms or more, with 596 of those weighing in at an incredible five kilograms, or 11lb.
At a regional level, women in the South West were most likely to have large babies, with 11.9% weighing at least four kilograms, followed by women in the South East, at 11.5%.
Women in London however were least likely to have large babies (7.7%). Overall, this meant big babies were more common in northern regions (10.4%) than in southern (10.1%) or midlands regions (9.8%).
The ONS does not publish data by local area.
NHS Digital however holds baby weight data for 283,600 births in England during 2021 (excluding May, for which data is missing) by council area.
Only babies born at 37 weeks gestation or later are counted in these figures, so premature babies with very low birth weights are excluded. Of the babies included in NHS Digital’s data, 10.9% were super-sized.
At a local level, it is a midlands council area that claims the crown for England’s biggest babies. In Rugby, a whopping 25.6% of babies weighed in at four kilograms, while 2.3% were five kilograms or more.
However, birth weight data was only recorded for one in every five of Rugby’s births, so the data may not show the true picture.
When looking at just areas where weight was recorded for at least half of births, West Devon moves into the top spot, with 21.6% four kilograms or heavier.
Mothers in Newham were the least likely to have large babies – excluding areas where the birth weight was not recorded in the majority of cases.
Just 160 out of 1,375 (5.5%) babies born in the East London borough weighed four kilograms or more. It was followed by Slough (5.8%) and Tower Hamlets (5.9%).
What causes macrosomia - and are there any risks?
Some mothers are more likely to have larger than average babies, including those who have diabetes or develop diabetes during pregnancy, or have a high Body Mass Index (BMI).
Professor Asma Khalil, spokesperson for the Royal College of Obstetricians and Gynaecologists, said: "While there is an increased risk of complications if a baby is [four kilograms], the majority of women do not have any complications and their baby is born safely without any problems."
One increased risk concerns shoulder dystocia, she said, where a baby’s shoulder gets stuck behind the pelvic bone during delivery. A mother will usually need extra help to free the baby.
She continued: "There is also an increased risk of the mother having heavier bleeding than normal after birth, and vaginal tears if having a vaginal birth.
"In some cases, if a woman had diabetes, they may be offered an early induction of labour, or a planned caesarean birth."
Are babies getting bigger?
In recent decades the average baby born in England has become heavier, according to a 2018 study published in the British Medical Journal.
Some anti-obesity campaigners have warned that unusually big babies are becoming increasingly common due to a rise in obesity among pregnant women.
Despite this, the data for each nation shows there has been no increased likelihood of newborns weighing four kilograms or more during the last decade across each UK nation.
In England, the proportion of super-sized babies has fallen from 11.4% in 2010 to 10.1% in 2020, the ONS data shows.