Obese or overweight women have a significantly elevated risk of developing health complications during pregnancy.
A joint study by teams at Queen's University and the Belfast Health and Social Care Trust monitored more than 30,000 mothers-to-be between 2004 and 2011.
Women who were overweight or obese were four times more likely to develop gestational diabetes during pregnancy, researchers found.
They were also three times more likely to have a stillbirth, premature delivery, or a newborn requiring neonatal care than women of normal weight.
Researchers followed 30,298 women, and found an estimated 2.8% were underweight, 52% were normal weight, 28% were overweight, 11% were obese class I, 3.9% obese class II and 1.9% obese class III.
Overweight women were more likely to need costly, specialist medical interventions, and also faced an increased risk of postnatal problems such as unsuccessful breastfeeding.
"Obesity rates have doubled in the last 30 years, co-author Dr Valerie Holmes, from the Centre for Public Health at Queen's University Belfast, said.
"This large-scale study clearly demonstrates that being overweight or obese during pregnancy increases the risk of adverse maternal and neonatal outcomes."
Copyright Press Association 2013