Pregnant smokers have been given a new deadline to quit.

Ground-breaking Auckland University research published in the British Medical Journal on Friday reveals pregnant women who smoke up to 15 weeks face no more risk of having a pre-term and small baby than if they had not smoked at all.

But if they continue to smoke beyond that period they are three times more likely to have a baby born pre-term, and twice as likely to have an underweight baby.

Smoking in pregnancy increases the risk of ectopic pregnancy, miscarriage, premature birth, stillbirth and neonatal deaths.

Babies born to smokers can also suffer a list of problems including cot death, development delays and respiratory diseases.

The lead author of the new research, Dr Lesley McCowan, an Auckland University associate professor in obstetrics and gynaecology, said the findings were of "considerable health importance".

She is talking about her findings for the first time at a conference in Auckland today.

She said the research showed for the first time women who stopped smoking in early pregnancy could reduce their risks of having a small or premature baby - about 8 per cent of Kiwi babies were born before 37 weeks.

The study surveyed 2500 women in New Zealand and Australia. The participants were divided into non-smokers, current smokers and those who stopped smoking before becoming 15 weeks' pregnant.

The results revealed no difference between the rates of premature births between stopped smokers and non-smokers, while smokers had much higher risk.