Why some pregnant women struggle with dangerous weight gain, while others add too little, could be partly explained by a three-year hormone study.

Scientists believe two major hormones involved in child-bearing - oxytocin and prolactin - may act as "on" and "off" switches for a pregnant woman's appetite.

During pregnancy, women must put on stores of fat to prepare for the demands of breast-feeding.

But Dr Colin Brown, a physiology researcher at Otago University, said more than 40 per cent put on too much weight - putting themselves and their babies at greater risk of birthing complications, and the children at increased risk of obesity and other health problems later in life.

Dr Brown, who received $820,000 from the Government's Marsden Fund for research, said oxytocin - the so-called love hormone that promotes bonding between a mother and child - suppressed appetite as well as carrying out vital functions such as triggering contractions.

It appeared that a spike in prolactin might block the appetite-suppressing function of oxytocin, allowing a woman to gain weight before she gave birth.

"For successful birth and successful lactation you have to have oxytocin secretion, so what we think what is happening is that the prolactin suppresses the ... anorectic effects of oxytocin over the course of pregnancy.

"Then sometime in the pregnancy ... the oxytocin cells escape from that inhibition to allow them to do the other functions.

"For normal birth you need oxytocin production."

Dr Brown said the two hormones spiked dramatically during pregnancy.

Prolactin developed the milk ducts ready for breast feeding, while oxytocin caused the ducts to contract and release milk.

Oxytocin had been shown to inhibit eating in virgin rats but not in lactating rats, said Dr Brown.

"It might be that increased prolactin stops the oxytocin from stopping you from eating ... What we're trying to do is work out exactly how that happens."

Dr Brown said finding a way to switch appetite on and off using hormones was still "pie in the sky".

But it was possible that different levels of prolactin in women soon to be mothers might predispose them to put on too much or too little weight.

Understanding how the two hormones interacted might help to pin-point which mothers were likely to be at greatest risk.

Dr Brown said the Marsden Fund money would allow him to employ a research fellow.