A study by researchers at the Universities of Edinburgh and Aberdeen and Information Services Division of NHS Scotland found women whose weight was above or below the norm required additional hospital care and incurred higher medical costs.
Those with a body mass index (BMI) above 30 and therefore classed as obese, cost £202.50 extra, while underweight women cost £102.30 more than normal-weight mothers-to-be. Part of the cost increase is down the duration and number of maternal admissions required after the birth.
Women with high or low BMIs are more likely to have complications giving birth, with severely obese women reporting a three-fold increased risk of hypertension and gestational diabetes.
More than 100,000 pregnant women were examined for the project and underweight women had an 8% increased risk for admission, while overweight, obese and severely obese women's risk grew 16%, 45% and 88% respectively.
Dr Fiona Denison, of Tommy's Centre for Maternal and Fetal Health,and co-author of the paper, said: "These findings further highlight the need for local and national government to implement strategies that will help reduce the prevalence of obesity.
"Longer term benefits of reducing maternal obesity will show improvements, not only in the health outcomes of mothers and their babies, but the workload and cost to current maternity services."