Study finds clusters of risk factors in pregnancy point to later problems.
A toddler is far more inclined to behave badly if exposed to more than three risk factors during pregnancy, such as the mother being a teenager, unemployed or financially stressed.
The latest report from the long-term Growing Up in New Zealand study found that 44 per cent of 2-year-olds from the high-risk category in late pregnancy scored as "abnormal" in a parental questionnaire that assesses the level of "negative behavioural outcomes".
High risk was defined as having four or more maternal risk factors from a list of 12, medium was one to three, and low risk was none.
The questionnaire asks parents things such as if the child is considerate of others' feelings, often has tantrums and has a good attention span. Only 8 per cent of toddlers from low-risk pregnancies were in the abnormal behaviour category, and 19 per cent from medium-risk pregnancies.
More than 5700 children were analysed for the study's second Vulnerability Report, made public today.
Study director associate professor Susan Morton, of the University of Auckland, said that across all risk categories 15 per cent were in the abnormal behaviour group based on the "strengths and difficulties" screening questionnaire, which is also used in the Government's Before School Check.
"They are potentially heading towards behavioural issues and problems with social behaviours."
She said the 44 per cent abnormal finding in the high-risk group was "much higher than we had anticipated".
She said the clustering and persistence of risk factors in association with potential behavioural issues asked: "How can we identify the most vulnerable children from birth and look at ways we might support them and their families, to mitigate against the effect of the developmental trajectories we are seeing even before age 2."
The questionnaire was answered for a second time at the 4-year-old checkpoint in the longitudinal study.
The study checked maternal risk factors at late pregnancy, and 9 months and 2 years old. At 2 years, 41 per cent were low risk, 46 per cent medium and 13 per cent high.
The most common risk factor was living in an area of high deprivation, experienced by at least 25 per cent of the children at the three measure points.