The saying that pregnant woman need to "eat for two" is a myth thought to have contributed to high rates of excessive weight gain.
A study involving three countries has found that 62 per cent of Auckland women put on excessive weight in their first pregnancy. The rate was 67 per cent in Adelaide, 80 per cent in Cork, Ireland, and 74 per cent across the three cities.
Compared with the 17 per cent of women in the study who had normal weight gain, the excessive weight gain group had an increased rate of caesarean births and a far higher rate of babies born large.
Excessive weight gain in pregnancy can also increase the risk of developing pregnancy-related diabetes, high blood pressure and the potentially fatal disorder pre-eclampsia. Pregnancy-related diabetes - which resolves after the baby is born - carries a 50 per cent chance of developing type 2 diabetes.
The kilos of pregnancy can be hard to shed after birth, contributing to compounding weight problems after multiple pregnancies - and big babies are themselves at ongoing risk of being overweight.
One of the study's authors, Professor Lesley McCowan of the University of Auckland, said other research in the Pacific Island communities of Auckland indicated the need to "eat for two" during pregnancy was a widely held belief.
In reality, only a little extra food was needed.
She highlighted Canterbury District Health Board guidelines stating in the first 12 weeks of pregnancy no extra food is required. After that, the necessary extra energy can be obtained by eating just one slice of wholegrain bread or two apples a day.
Pregnancy adds to a woman's body in many ways - in addition to the fetus - such as a larger uterus and breasts, a placenta, amniotic fluid, and more blood and fat.
A woman who is at a healthy weight for her height should put on 11kg to 16kg while pregnant with a single baby. The optimum gain is a little more for those who are underweight, a little less for those overweight and less again for those who are obese. However, the amounts vary depending on each woman's weight and height, which should be discussed with her midwife or doctor.
Professor McCowan said: "One of the things we were really shocked about was 74 per cent of these so-called low-risk women - in their first pregnancy - had an excessive weight gain. It's a really worrying statistic."
The study's Auckland women were mainly Europeans so ethnic comparisons weren't possible, but other data sources revealed a vast divide. The Counties Manukau DHB found at the time maternity care was booked, 61 per cent of Pacific women were obese, in contrast to 38 per cent of Maori and 22 per cent in the European/other category.
Professor McCowan is preparing South Auckland research on culturally appropriate dietary interventions to help Pacific women gain a healthy amount of weight in pregnancy.
Read more: Mum at loss to explain weight gainBy Martin Johnston Email Martin