About 20 per cent of women continue to drink alcohol after finding out they are pregnant - with some reporting having four or more drinks a week.
The results come from a new study released today looking at pregnant women's behaviour regarding alcohol.
Up to 6800 expectant mums from around New Zealand were surveyed for the Changes in alcohol consumption during pregnancy report by the Social Policy Evaluation and Research Unit (Superu) using data from the Growing Up in NZ study.
The women were asked a series of questions relating to drinking beer, wine or spirits before and after they knew they were pregnant.
Figures show up to 71 per cent of women consumed alcohol before becoming aware they were with child.
That dropped to 21 per cent drinking during the first trimester, while 13 per cent of women said they continued to drink after the first trimester.
The research unit's chief executive, Clare Ward, said the results indicated a need to provide more information to expectant mothers about the effects of drinking alcohol during this time.
"Better understanding of women's alcohol consumption during pregnancy will help inform and target public health messages and support initiatives."
A small percentage (2 per cent) of women drank throughout their full term and the study showed they tended to be mums aged 35 or older, Maori, had a low income or smoked.
Women (29 per cent) who abstained from alcohol altogether tended to be of Asian or Pasifika descent, did not smoke or were from a low-income household.
The Ministry of Health guidelines give clear warnings to women who drink alcohol while pregnant and indicate there is no known safe level of alcohol consumption at any stage of pregnancy.
Among the warnings is that expectant mums who drink are more likely to give birth to babies who are smaller, premature or have developmental, behavioural or physical problems known as fetal alcohol spectrum disorder, or FASD. There is also the risk of miscarriage.
Claire Gyde , chairwoman for the Fetal Alcohol Spectrum Disorder Care Action Network, said women who chose to have a few drinks while pregnant were putting their child at a huge risk.
"It needs to be a zero tolerance policy. Women should be advised that there's no safe amount.
"It doesn't matter who you speak to, lots of women believe if you have the odd drink, it's not going to cause any harm - and it maybe it doesn't with your child," she said.
"But it's like Russian roulette. You just don't know."
71% of women consumed alcohol prior to awareness of pregnancy.
21% consumed alcohol during first trimester.
13% continued drinking after first trimester.
2% of women continued drinking throughout pregnancy.
Source: Social Policy Evaluation and Research Unit
'No one talked about the risks'
Teenager Alex Prichard is a happy kid who loves playing video games and can school anyone in the game of Minecraft.
But the 13-year-old struggles some days because of behavioural issues related to having fetal alcohol spectrum disorder.
The teen, from Napier, was diagnosed when he was 8.
Mum Jackie Prichard, who was 24 when she was pregnant with Alex, said she simply did not know about the risk to her baby by having the odd drink.
"We never talked about it at school or at the doctors ... they hounded me about smoking, but there was no mention of alcohol."
Mrs Prichard, who was a smoker, cut back on cigarettes and took it upon herself to cut back on alcohol, but said she had a few drinks every week or so socialising with friends.
Alex has two younger sisters - both of whom were born with no issues as their mum had by then stopped drinking alcohol altogether.
Mrs Prichard said she wanted to tell all expectant mothers to not take the risk.
"All [medical professionals] across the board need to say no drinking during pregnancy. That's it." She admitted there were difficult days when it came to her son, but said she was the first person to pull him up again. "When things are out of his control, when ... he loses grasp of the world around him and starts to lose it, that's when it often hits me.
"I usually wait when I'm away from the kids and I might have a little cry. But I don't dwell on it because I have to help him through because no one else is going to."