Obese women have a higher risk of suffering complications when they are giving birth, a study has found.
Women are advised to lose weight before they become pregnant
The Cardiff Birth Study looked at 8,350 women who were having their first baby, a third of whom were obese.
Those who were obese faced a higher risk of being overdue, having their labour induced or a Caesarean section.
Experts said the British Journal of Obstetrics and Gynaecology study raised concerns, and advised women to lose weight before becoming pregnant.
Being obese is classed as having a body mass index (BMI) score of over 30.
BMI is calculated by dividing a person's weight in kilograms by the square of their height in metres.
The incidence of pregnancies involving obese women has more than doubled in the past 10 years.
A third of the pregnant women studied by the Welsh researchers, who all gave birth between 1990 and 1999, were obese.
In addition to being less likely to have a natural delivery, these women had a higher chance of experiencing maternal blood loss, urinary tract infections and of the baby's shoulders getting stuck during vaginal delivery.
Among those who underwent Caesareans, emergency procedures were more likely than planned procedures.
In addition, more babies born to obese women required admission to the neonatal unit and were more likely to require assistance with feeding and maintaining body temperature.
Dr Talakere Usha Kiran, who led the study, said it was not clear why obese women had this increased risk.
"We have found the problem. Now we need to go about identifying a solution."
But she said the risk of complications could be reduced if it could be discovered why the women went past their delivery date - and this could be prevented.
"When women are overdue, they are likely to be induced or to have to undergo a Caesarean."
Mr Peter Bowen-Simpkins, spokesman for the Royal College of Obstetricians and Gynaecologists, said: "Women with a BMI of over 30 were more at risk of problems both during labour and afterwards and should be regarded as high risk.
"Babies are more difficult to deliver; the women are more likely to have a Caesarean - and these are more difficult because of the challenge of cutting through the abdominal fat."
Mr Bowen-Simpkins said women should lose weight before they tried to become pregnant.
Not only did obesity raise the risk of birthing complications, it also reduced the risk of becoming pregnant in the first place.
He said: "It's a good idea all round."