Obese or overweight women are more likely to develop life-threatening health complications during pregnancy, research has revealed.
Their risk of gestational diabetes was increased four-fold and they were also three times as likely to have a stillbirth; premature delivery, or a newborn requiring neonatal care as women of normal weight.
Consequently overweight women are more likely to need costly, specialist medical care when pregnant.
"Obesity rates have doubled in the last 30 years. This large-scale study clearly demonstrates that being overweight or obese during pregnancy increases the risk of adverse maternal and neonatal outcomes," said Dr Valerie Holmes from the Centre for Public Health at Queen's University Belfast, who co-wrote the report.
The team from Queen's University and the Belfast Health and Social Care Trust monitored more than 30,000 mothers-to-be over an eight-year period. For the first time women were categorised according to the World Health Organisation body mass index (BMI) classifications - underweight; normal weight; overweight; plus three obese sub-categories.
"By having obesity in sub-classifications, we were able to highlight the relationship between increasing BMI and the increasing risk of adverse outcomes, with women most at risk in obese class III requiring specialist medical care during pregnancy," added Dr Holmes.
Of the 30,298 pregnancies assessed between 2004 and 2011, an estimated 2.8% of women were underweight, 52.% were normal weight and 28% were overweight. A further 11% were obese class I, 3.9% obese class II and 1.9% obese class III.
In overweight and obese women there was also an increased likelihood of postnatal problems, such as unsuccessful breastfeeding. Conversely, underweight women risked developing anaemia and were more likely to have a low birth weight baby.
Dr Dale Spence from the School of Nursing and Midwifery at Queen's University Belfast said new guidelines were needed to manage the care for overweight women.
He said: "We found that the majority of overweight women fall into the overweight or obese class I categories and while they are still at an increased risk of gestational diabetes and hypertensive disorders in pregnancy, they may not be offered the same level of specialist care under current guidelines."