What determines the birth weight of a baby?

Washington: Recent findings have discovered links between genetic code and birth weight of human beings which explain how mothers' and babies' genes influence birth weight.

Scientists have long known that babies who are particularly small at birth have a higher risk of birth complications, and also tend to be more prone than average weight babies to high blood pressure in adulthood. Findings of the study were published in the Journal of Nature Genetics.

As part of the study, the researchers looked at genetic information from 230,069 mothers, with the birth weight of one child each, in addition to genetic information and birth weights of 321,223 people across the UK Biobank and the Early Growth Genetics consortium cohorts.

A child inherits half their genes from their mother and a half from their father, and the child's own resulting genetic make-up plays a role in birth weight. The paper reveals the complex balance of how both the mother's genes and the baby's genes can influence the baby's growth.

The researchers concluded that the direct effects of a baby's genes made a substantial contribution to birth weight. However, around one-quarter of the genetic effects identified were from the mother's genes that were not passed on to the child. Instead, these affected the baby's growth by influencing factors in the baby's environment during pregnancy, such as the amount of glucose available.

The study found that some parts of the genetic code can be linked to birth weight both directly from the child and indirectly from the mother. A number of these were seen to work together, with the mother and baby effects pushing birth weight in the same direction, while others had opposing effects, like a mother-baby tug of war.

For example, some of the genetic effects that raise the mother's glucose levels work to make the baby bigger because the baby produces more insulin in response which makes it grow. But when those same variations in the genetic code are inherited by the child, they restrict the amount of insulin the baby can produce, so limiting its growth and counter-acting some of the mother's growth-promoting effects.

‘This is the first time we've really been able to unpick the effects of both mother and baby's genes on baby weight, which is an important health indicator. It's particularly useful to know about the maternal genetic influences on the environment in the womb because these give us clues as to which factors are causal. Better understanding of the causes may mean we can help ensure babies are born at healthy weights,” said Rachel Freathy, one of the lead authors on the study.

According to the researchers, this study highlights the value of large-scale international research collaborations.