Almost half of all mums-to-be put on too much weight during pregnancy while more than a fifth don't gain enough, a Monash University study shows.

Three quarters of women either gain too much or too little weight during pregnancy, Australian researchers have found after analysing data gathered from more than one million mums-to-be around the globe.

Their findings have sparked calls for women to be weighed more often during pregnancy and given more information from health professionals about just how much weight they should gain.

Researchers from Monash University's Faculty of Medicine found three-out-of-four women don't gain a healthy amount of weight during their pregnancies, putting themselves and their babies at risk of developing health problems.

Almost half put on too much weight, while more than a fifth didn't gain enough.

At the start of their pregnancies, just over half of the women had been a healthy weight while 38 per cent were overweight and seven per cent underweight.

The Monash researchers, headed by Professor Helena Teede and Dr Rebecca Goldstein, based their findings on a fresh analysis of data from 1.3 million pregnant women who took part in more than 5300 studies in Europe, Asia and the United States.

"The findings strongly support the fact that we need to do more to help women go in to pregnancy at a healthy weight and gain a healthy amount of weight in pregnancy so they have less complications," Prof Teede told AAP.

"Basically it's not about eating for two. For the first trimester there should be no increased calories, the second trimester is about 330 calories extra a day and third trimester is about 450 calories a day (extra).

"Women also don't need to be 'confined', they need to remain active."

Pregnant woman who don't put on enough weight are at risk of delivering smaller babies and going into labour early.

Those who gain too much weight can are more likely to have bigger babies and need a caesarean.

The Monash study has brought into question the relevance of guidelines for healthy weight gain in pregnancy developed in the US four decades ago.

Prof Teede supports a review of current guidelines, as well as the introduction of frequent weigh-ins for mums-to-be, and more training for health professionals so they can better inform pregnant women about healthy weight gain in pregnancy.

In Australia, women tend to only be weighed at the beginning of their pregnancy and only measured again if medical staff think it's warranted.

"The point is with this data is that we should be weighing all of them given that three in four don't gain a healthy amount of weight," Prof Teede said.

"And we need to provide them with simple support to improve their lifestyle.

"But it's really important not to say to mums you have to deprive yourself and if you're hungry you shouldn't eat.

"It's about just eating really healthy foods, just follow the good ol' food pyramind, it's not complicated.

The Monash study was published in the Journal of the American Medical Association on Wednesday.

CURRENT GUIDELINES FOR HEALTHY WEIGHT GAIN DURING PREGNANCY

• Underweight women should gain 12.5-18kg
• Healthy weight women should gain 11.5-16kg
• Overweight women should gain 7-11kg
• Obese women should gain 5-9kg

(Source: Department of Health)