The trend for new mothers to lose lots of weight in a short amount of time could be detrimental to their infant’s health, they warned.
But women who pile on the pounds after giving birth also put their next child at risk, they said.
Effects can include premature birth, a greater risk of stillbirth and high blood pressure.
Women should therefore try to maintain a healthy weight before, during and after pregnancy to give their child the best start in life, said two doctors writing in the British Medical Journal.
The experts are Jennifer Walsh, a registrar in obstetrics and gynaecology at Coombe Women’s Hospital in Dublin, and Deirdre Murphy, professor of obstetrics at the hospital and Trinity College, Dublin University.
They warned that the current obsession with what women weigh and its series of conflicting messages also extended into pregnancy.
They say: “Pregnancy is one of the most nutritionally demanding periods of a woman’s life, with an adequate supply of nutrients essential to support foetal wellbeing and growth.
“With at least half of all pregnancies unplanned, women need to be aware of the implications of their weight for pregnancy, birth, and the health of their babies.
“However, the potential to provide women with conflicting information about weight, weight gain, and weight loss extends to pregnancy and birth outcomes.”
The authors cited two studies which show the effects of weight gain and weight loss.
The first, a Swedish study, involved 207,534 women. The second was in the American Journal of Obstetrics and Gynaecology.