Inflammatory bowel disease doubles the risk of pregnancy complications, a study says.
Over 300,000 women were researched in 12 studies
Researchers found women with disorders such as Crohn's disease and ulcerative colitis were more likely to have underweight and premature babies.
The team from St Mary's Hospital in London analysed 12 studies published over the last 20 years, the Gut journal reported.
But experts said modern treatments meant the risks were now low.
The studies analysed involved almost 4,000 women with either Crohn's disease or ulcerative colitis and more than 320,000 people without such conditions.
The results showed that women with the disorders were almost twice as likely to have a child born prematurely, and more than twice as likely to have a child born below normal weight.
Premature and low birthweight babies run the risk of developmental problems and a greater likelihood of serious chronic illness.
Women with the disorders were 1.5 times as likely to have had a caesarean section as their healthy peers.
And the rate of congenital birth defects in babies born to mothers with the disorders was more than twice as high.
The researchers were not able to explain what caused the increased risk.
But they said if a woman becomes pregnant during an active bout of disease the likelihood of pregnancy complications would probably be greater.
Co-author Dr Paris Tekkis said pregnant women with such disorders should be considered as an at risk group.
"The surgeon and obstetrician need to discuss between themselves the management of delivery in women with IBD.
"A definitive study is required to settle the issue of best management and from this a new set of guidelines, to help both patients and their clinicians determine best practice."
Dr Maggie Blott, of the Royal College of Obstetricians and Gynaecologists, said pregnant women with these disorders would receive special care.
But she added in her experience women with these disorders were not suffering from a disproportionate number of complications.
"They do elect to have caesareans so that would explain why that figure is higher, but I do not think in the present day the rates for other things are higher.
"Women with the conditions often choose to have children when their disease is well under control."
She suggested that the data from the older studies may have distorted the findings.
"Treatment has been improving, but in the past maybe babies were not getting all the nutrients they needed."