Overweight mothers have larger babies: study

Mothers who are overweight or obese during pregnancy give birth to larger babies, according to a new study.

The research, led by the universities of Exeter and Bristol in the UK, also found that having higher blood glucose (sugar) during pregnancy causes babies to be born larger.

Conversely, having higher blood pressure in pregnancy causes babies to be smaller.

Scientists have shown that excess weight and higher glucose levels in mothers cause their babies to be born heavier, while higher blood pressure causes lower birth weights. They found that mothers with higher blood sugar tend to have bigger babies, even within a healthy range.

Unlike some previous studies, the research also found that mothers' blood lipids (levels of fat) that are also related to being overweight did not seem important in determining the baby's size.

Researchers used data from more than 30,000 healthy women and their babies across 18 studies. They examined genetic variants associated with mothers' body mass index, blood glucose and lipid levels and blood pressure, along with measurements of those characteristics in pregnancy.

They also studied the weight of all the babies at birth. All the women had European ancestry and were living in Europe, US or Australia. Babies born between 1929 and 2013 were included in the study.

"A lot of research into pregnancy and birth weight has been based on observation, but this can make it very difficult to determine what is cause and what is effect, creating a confusing picture for mothers, clinicians and health-care workers," said Jess Tyrrell from University of Exeter.

"Our genetic method is more robust, giving clear evidence that mothers' weight, glucose and blood pressure affect the size of the baby," said Tyrrell.

Even though being overweight or obese is usually associated with having a higher blood pressure, researchers found that higher blood pressure causes babies to be born smaller, suggesting that there are complicated factors affecting growth in the womb.

The findings were published in the JAMA journal.