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.