Small and large birth weight linked to genetics of mother and baby – except in tiniest babies, study suggests

A large-scale study by Cardiff University and the University of Exeter has found the strongest evidence to date that genetics play a major role in most cases when babies born at full term are in the top or bottom 10% of the birth-weight spectrum.

However, in the 3% of babies with the smallest birth weights, genetics seemed to play a less important role, the researchers found, indicating that other factors may be contributing to the babies’ small size.

Researchers looked at 190 common genetic variations that are known to affect birth weight. Other important factors could include the health of the mother or fetus or of the placenta, which transfers nutrients and oxygen to the baby.

Co-lead author and clinician, Professor Sailesh Kotecha, from Cardiff University’s School of Medicine, said: “It’s important to identify reasons why babies are born with low birth weight as they are at risk of increased health problems in later life, including diabetes and high blood pressure."

To examine the extent to which birth weight was linked to the genetics of mothers and babies, the team created a genetic score for higher birth weight.

The study, published today in PLOS Genetics, tested whether the genetic score was higher or lower in babies who were born very large or very small in a sample of nearly 12,000 babies and more than 5,000 mothers of European ancestry.

The weight of babies at birth is important, as those born at the extreme ends are at higher risk of complications. Smaller babies are more likely to be admitted to neonatal units and at higher risk of death, while larger babies are at higher risk of complications during birth.

Dr Robin Beaumont, of the University of Exeter Medical School, co-lead author of the study, said: “This research casts new light on why some babies are born very large or small. This knowledge will help both parents and clinicians understand where they need to focus medical attention.

“Genetics played a lesser role in the 3% of babies with the lowest weight, suggesting that other factors such as the health of the placenta, may have influenced their weight.”

The researchers said it was also possible that rare genetic changes in the baby could reduce growth in the smallest 3%.

Professor Rachel Freathy, of the University of Exeter, who oversaw of the study said the study “gives the greatest insight to date into how common genetic variations between people influence the extremes of birth weight”.

“We now need to understand better whether the genetics or environmental factors are more important in the later life health risks,” she said.

The University of Bristol, Imperial College London and the University of Oulu in Finland also collaborated on the study. The mothers and babies were from Children of the 90s, a health study based at the University of Bristol, the Exeter Family Study of Childhood Health and the Northern Finland Birth Cohorts 1966 and 1986.

The research was supported by Wellcome Trust and the H2020 program of the European Commission.