A $5 million a year study has found that 40 per cent of the babies born in Auckland and the Waikato in the past year were "accidents".

The taxpayer-funded 21-year study, which will follow 7000 babies until they reach adulthood, has pinned down the proportion of unplanned babies more accurately than any other long-term study because it started questioning mothers and fathers about three months before their babies were born.

The study also found that:

* Widespread intermarriage means that almost half of the babies in the study area have multiple ethnicities.

* Two out of five babies are born in the poorest third of the region, and half are in families that don't own their own homes.

* More than a third of first-time mothers do not know about family tax credits that could give them an extra $86 a week.

The study aims to find the factors that help some children do better in life than others, and to answer policy-related questions such as why some pregnant women who plan to vaccinate their babies end up not doing so.

The finding that 40 per cent of births are unplanned has huge implications because unborn babies may be harmed by the mother's drinking, eating and smoking before she realises she is pregnant.

Almost a third (31 per cent) of the women who fell pregnant by accident drank alcohol during the first three months of their pregnancy, compared with only 17 per cent of the women whose babies were planned.

"We will be able to track how those patterns of drinking before and during pregnancy are reflected in birth outcomes and early development," said study director Dr Susan Morton of Auckland University.

"That is novel internationally."

The Health Ministry recommends women not drink at all while pregnant because of the risk of fetal alcohol spectrum disorder.

But this message was eroded by a widely-criticised British study in October which linked light maternal drinking in pregnancy with a lower rate of social or emotional difficulties in the children than in the children of those who quit alcohol in pregnancy.

Dr Morton said the proportion of unplanned babies was slightly lower than the Health Ministry's previous "best guess" of around half, but it still showed the importance of informing young women and girls about healthy diet and lifestyles.

The study found that 58 per cent of women who had planned their pregnancies were taking folic acid supplements before they conceived, against 8.9 per cent among the unplanned pregnancies.

Others started taking the supplements during pregnancy, but 8 per cent of the planned group and 28 per cent of the unplanned, did not take them before or during pregnancy.

The ministry urges women to take folic acid supplements from four weeks before conception and for 12 weeks afterwards, to reduce the risk of some birth defects including spina bifida.

The first findings of the study, Growing Up in New Zealand, were published yesterday. They reveal the diversity of babies being born in Auckland and the Waikato. Almost half (44 per cent) have multiple ethnicities, and a third of the mothers were born overseas.

* 6822 babies born in Auckland, Counties-Manukau and Waikato between February 2009 and June 2010.

* 6822 mothers recruited through midwives, advertising and malls.

* 4404 partners for whom mothers provided contact details.

* 99 per cent of participating partners are biological fathers of the babies.

* All will be followed for 21 years.

* Study aims to find why some children do better in life than others.

* It is costing $5 million a year.