Mothers who gain or lose lots of weight between pregnancies could be putting their baby at risk, say experts.
Being too fat or thin puts mother and baby at risk, say experts
Fluctuating weight ups the risk of high blood pressure and type 2 diabetes in the mother and the chance of stillbirth, research suggests.
The work by Dublin-based specialists is in the British Medical Journal.
They said pregnancy was "one of the most nutritionally demanding periods of a woman's life" and women needed to be aware of the implications of weight.
The work was authored by Jennifer Walsh, a specialist registrar in obstetrics and gynaecology at Coombe Women's Hospital, Dublin, and Deirdre Murphy, professor of obstetrics at Trinity College, University of Dublin.
Women of normal weight are often advised to avoid piling on the pounds between pregnancies and obese women are told to shed excess pounds before pregnancy.
But the Dublin specialists said women were being bombarded with mixed messages about diet, weight and body image, which were extending into pregnancy.
They said: "There is growing concern on the one hand about an epidemic of obesity, and on the other about a culture that promotes 'size zero' as desirable, irrespective of a woman's natural build.
"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.
"Women are at an increased risk of different but equally serious adverse pregnancy outcomes if they gain or lose an excessive amount of weight between pregnancies."
They point to two studies.
The first, from Sweden and involving 207,534 women, found that weight gain between pregnancies was strongly associated with major complications for the woman and baby in the months preceding, during and just after childbirth.
The second found that women whose weight fell significantly between pregnancies had a higher risk of giving birth prematurely than women whose weight remained stable or increased.
Tam Fry, of the National Obesity Forum, said: "Women should be aiming for a normal weight before they have their second child.
"But this research also shows that women also go the other way and starve themselves to plummet to a goal weight. That is also wrong."
He said young girls should be taught the importance of maintaining a healthy weight, not only for their own physical wellbeing but also for that of any children they may want to have in the future.
"There is a known association between overweight and obese parents and the likelihood of a child being overweight themselves," he added.