Being overweight or obese in pregnancy causes babies to be born larger, according to new research.
The study, led by the Universities of Bristol and Exeter, also found that mothers with a higher blood sugar - even within a healthy range - tend to have bigger babies.
However, having higher blood pressure in pregnancy causes babies to be born smaller, the international research collaboration found.
Researchers found that mothers' blood lipids, which are also related to being overweight, did not seem important in determining the baby's size.
Dr Rachel Freathy, of the University of Exeter 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 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 work, published in medical journal JAMA, 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.
They also studied the weight of all the babies at birth.
All the women in the study had European ancestry and were living in Europe, America or Australia. Their babies were born between 1929 and 2013.
Dr Jess Tyrrell, of the University of Exeter 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 but the researchers found this was linked to babies being born smaller.
Professor Debbie Lawlor, from the University of Bristol'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, and we are grateful to everyone involved.
"We will continue to work together 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 themselves - do children born to women with high glucose levels in pregnancy continue to be heavier throughout their lives?"
The research was supported by the Wellcome Trust, Royal Society, the Medical Research Council, the US National Institute of Health, the European Research Council and the Diabetes Research and Wellness Foundation.