Small and large birth weight linked to genetics of mother and baby - except in tiniest babies

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.