Rising obesity trends mean that by 2010, half of all UK mums-to-be could be above ideal weight and a fifth obese, experts warn.
A healthy weight is important for mum and baby
Women from deprived areas run the greatest risk and are twice as likely to be obese at the start of pregnancy.
Urgent action is needed to halt this "serious public health time bomb," a team from Teesside wrote in the journal BJOG.
They said it was a problem that had not been fully appreciated until now.
Researchers from the North East Public Health Observatory studied nearly 37,000 women at a local maternity unit over 15 years.
Maternal obesity had risen from 9.9% in 1990 to 16% in 2004, they found.
If this trend continues, by 2010, 22% of pregnant women will be obese, putting a strain on maternity services, researcher Nicola Heslehurst and colleagues warn.
Being overweight during pregnancy is a big health risk for both mother and baby.
Obese mums-to-be are more likely miscarry, experience pre-eclampsia and dangerous blood clots or need a Caesarean section to deliver the baby, which is likely to be larger itself.
According to Confidential Enquiries into Maternal and Child Health, obesity is a feature of 35% of maternal deaths.
Babies of obese mothers also face a higher death risk.
Professor John Wilkinson, director of North East Public Health Observatory, said: "Maternal obesity is something that has crept up on us.
"We had anecdotal evidence and were aware that heavier women were coming in to book a pregnancy, but we needed some hard evidence."
"Public health time bomb"
Professor Carolyn Summerbell who heads up the Centre for Food, Physical Activity, and Obesity research at the University of Teesside said: "Future research programmes aimed at preventing the continuation of this trend are imperative."
Professor Phil Steer, editor-in-chief of BJOG, agreed saying: "This is a serious public health time bomb and of major anxiety to healthcare professionals and providers.
"At a time when NHS resources are stretched, the scale of the problem in future years will need to be scoped, to identify 'at-risk' groups and help plan service delivery more effectively."
Gail Johnson of the Royal College of Midwives said: "Socially excluded groups already face challenges in maintaining health and obesity may make health gains harder to achieve.
"We would encourage women who are concerned about weight gain to talk to their midwife to identify what help and advice is available and to take the issue seriously."