The study, published Tuesday in the Canadian Medical Association Journal, challenges a growing consensus in the scientific community that elevated glucose levels in the mother are primarily responsible for overly large babies, a condition known as macrosomia.
Researchers at four Toronto health institutions conducted the study in response to a growing trend among medical professionals treating women for gestational diabetes, a temporary form of the disease only contracted during pregnancy.
Lead researcher Ravi Retnakaran of Mount Sinai Hospital said some scientists were uncomfortable with proposed new guidelines that would lower the minimum glucose threshhold for identifying the condition, which would lead to a sharp spike in the number of diagnosed cases.
Retnakaran's team joined forces with researchers at the Hospital for Sick Children, St. Michael's Hospital and the University of Toronto to evaluate which factors were the strongest predictors of infant macrosomia.
Scientists monitored 472 women to gauge the effect of maternal weight, blood sugar and lipid levels on the size of a newborn child.
Retnakaran said the results suggested that glucose was much less of a factor in predicting infant weight than previously believed.
Instead, he said, women who were overweight before pregnancy were more likely to give birth to an infant deemed large for gestational age, or weighing more than four kilograms. The risk was almost equally great for women who gained excessive weight during their pregnancies.
Retnakaran said the findings should make medical practitioners and expecting mothers alike reconsider their priorities.
"We think that optimizing maternal health, including maternal weight before the pregnancy, is probably the most important thing that can be done to reduce the risk, not only of gestational diabetes, but also that risk of the baby growing too big," Retnakaran said in a telephone interview.
Reducing the risk of infant macrosomia has wide-spread benefits for both mother and child, he said. Overly large babies can cause complications during pregnancy and pose delivery challenges at birth, greatly increasing the need for a cesarian section.
Babies don't benefit from growing too large in the womb either, Retnakaran said, adding abnormal growth as a fetus can have long-term implications as an adult.
"There are relationships between a child's birth weight and their risk of developing diabetes and heart disease 50 years later," he said. "Birth weight is a marker of the inter-uteran environment, and what's determining that risk of disease five decades later is the fetal programming that's going on in the face of that environment."
The study findings echo similar research from the Children's Hospital of Eastern Ontario released last month.
That data, published in the online issue of the Journal of Maternal Fetal and Neonatal Medicine, found that excessive weight gained during pregnancy increased the chance of producing a large baby regardless of whether the mother herself was overweight.
Retnakaran and Cheo researchers both concluded women should do all they could to regulate their weight both before and during their pregnancies.