The study, published in the Journal of the American Medical Association, also found that mothers with higher blood sugar tend to have larger babies, even within a healthy range.

Conversely, having higher blood pressure in pregnancy causes babies to be smaller, according to the study led by the universities of Exeter and Bristol.

Overweight mothers tend to have bigger babies: <a href=http://www.hxen.com target=_blank class=infotextkey>study</a>

"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," Rachel Freathy, of the University of Exeter Medical School, who co-led the study, said in a statement.

"Higher and lower birth weights are also associated with diseases such as Type 2 diabetes later on 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 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 sugar, lipid or fat levels and blood pressure, along with measurements of those characteristics in pregnancy and the weight of all the babies at birth.

All the women had European ancestry and were living in Europe, America 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 healthcare workers," Jess Tyrrell of the University of Exeter Medical School, co-lead author of the paper, said.

"Our genetic method is more robust, giving clear evidence that mothers' weight, glucose and blood pressure affect the size of the baby."Interestingly, even though being overweight or obese is usually associated with having a higher blood pressure, the researchers found that higher blood pressure causes babies to be born smaller, suggesting that there are complicated factors affecting growth in the womb.

In addition, the research found that mothers' blood lipids, or levels of fat, which are also related to being overweight, did not seem important in determining the baby's size.

Next, the researchers will work to answer the next important question about 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 themselves.