Health experts have warned that, contrary to the popular myth, pregnant women do not need to "eat for two".
The advice has been published in a patient leaflet from the Royal College of Obstetricians and Gynaecologists (RCOG) amid concerns about the growing numbers of women who are overweight or obese at the start of pregnancy.
The new guidance urges overweight patients to stick to a healthy diet and take some exercise.
Experts say that "eating for two" is not necessary as women do not need any extra calories until they are in their third trimester, and even then only an extra 200 a day, which equates to two pieces of toast.
The guidelines state that while the majority of women who are overweight (with a body mass index (BMI) 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.
Philippa Marsden, Chairman of the RCOG’s Patient Information Committee, said, as cited in the Telegraph: "This new advice gives women information about the risks of being overweight during pregnancy and giving birth but more importantly focuses on how women can work together with their health care team to reduce these risks. It also details the extra care a woman should have during her pregnancy if she has a raised body mass index.
"Trying to lose weight by dieting during pregnancy is not recommended – even if you are obese – as it may harm the health of your unborn baby. However, by making healthy changes to your diet you may not gain any weight during pregnancy and you may even lose a small amount.
"Healthcare professionals can also help you with losing weight if you are planning for a future pregnancy or after you have had your baby."