Mothers who are overweight or obese during pregnancy give birth to larger babies, new research claims.
The study, led by Bristol and Exeter Universities, found that mothers with higher blood sugar – even within a healthy range – also tended to have bigger babies. By contrast, having higher blood pressure during pregnancy causes babies to be born smaller, the international research suggested.
Researchers found that mothers’ blood lipids, or levels of fat – altered levels of which are also related to being overweight – did not seem important in determining the baby’s size.
Dr Rachel Freathy, of Exeter University medical school, said: “Being born very large or very small can carry health risks for a newborn baby, particularly when that’s at the extreme end of the spectrum.
“Higher and lower birth weights are also associated with diseases such as type 2 diabetes later in life.
“Understanding which characteristics of a mother influence the birth weight of her offspring may eventually help us to tailor management of a healthy pregnancy, and reduce the number of babies born too large or too small.”
The work, published in the Journal of the American Medical Association, used data from more than 30,000 healthy women and their babies across 18 studies.
Researchers examined genetic variants associated with mothers’ body mass index, blood glucose and lipid levels, and blood pressure, along with measurements of these characteristics in pregnancy and the weight of all the babies at birth.
All the women in the study had European ancestry and were living in Europe, the US or Australia. Their babies were born between 1929 and 2013.
Dr Jess Tyrrell, of Exeter University medical school, said: “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 healthcare workers.
“Our genetic method is more robust, giving clear evidence that mothers’ weight, glucose and blood pressure affect the size of the baby.”
Being overweight or obese is usually associated with having a higher blood pressure and the researchers found this was linked to babies being born smaller.
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Prof Debbie Lawlor, from Bristol University’s school of social and community medicine, added: “This is really important research that could only be done with collaboration from a large number of scientists and the involvement of participants from several countries.
“We will continue to work to answer the next important question, which is whether the effects of mothers’ weight, glucose and blood pressure on their babies’ weight at birth has a lasting effect as their children grow and become adults. Do children born to women with high glucose levels in pregnancy continue to be heavier throughout their lives?”