Overweight women are being warned by doctors not to “eat for two” in pregnancy.
The Royal College of Obstetricians and Gynaecologists urge heavier patients to stick to a healthy diet and take some exercise.
Experts have become increasingly concerned in recent years about the growing numbers of mothers-to-be who are overweight and obese at the start of pregnancy.
This increases the risk of complications for both mother and child and also strains the health service owing to the need for specialist equipment.
A guide published by the RCOG says that while the majority of women who are overweight (with a body mass index greater than 25) will have a straightforward pregnancy and birth, the risk of complications goes up the heavier a woman is.
Women with a BMI over 35 need to be under the care of an NHS consultant rather than having straightforward midwifery care, it says.
Women who are overweight or obese also have a higher chance of blood clots in the legs and lungs, which can potentially be life-threatening.
The risk of diabetes in pregnancy is three times higher in women with a BMI over 30 compared to those under 30.
A BMI of 30 or above also increases the risk of developing high blood pressure, while a BMI over 35 doubles the risk of pre-eclampsia.
Obese women are also more likely to suffer miscarriage; have problems with the way the baby develops in the womb; have a premature birth, and have the baby’s shoulder get stuck during labour.
A longer labour is also more common in obese women, who are also more likely to need an emergency caesarean section, the guidance says.
It adds that while the overall risk of stillbirth in the UK is one in 200, this doubles to one in 100 among women with a BMI over 30.
The guidance on diet urges women to base their meals on starchy foods such as potatoes, bread, rice and pasta, choosing wholegrain where possible.
“Watch the portion size of your meals and snacks and how often you eat,” it adds. “Do not ‘eat for two’.”
Women are advised to eat as little as possible of fried food, drinks and sweets high in added sugars, and other foods high in fat and sugar.