If you're a smoker and planning to get pregnant, there's good news and bad news.

First the good news: Australian researchers say women who stop smoking before 15 weeks of pregnancy cut their risk of giving birth prematurely and having small babies to the same level as non-smokers.

Now the bad: Women who don't quit by 15 weeks are three times as likely to give birth prematurely and twice as likely to have small babies, compared to women who stopped smoking.

The lesson, says lead author Dr. Lesley McCowan from the University of Auckland, is that that maternity care providers need to emphasize the importance of mothers-to-be giving up smoking as early as possible.

The findings are based on a study of 2,500 healthy women, 80 per cent of whom were non-smokers and 10 per cent of whom had stopped smoking before 15 weeks. The remaining 10 per cent smoked past 15 weeks.

Timing critical to reduce risk

Doctors already know that when pregnant women stop smoking their likelihood of premature births and delivering low-birth weight babies decreases, but this is one of the few studies to examine whether there is a critical time by which smoking must stop to reduce such risks.

Another important finding was that women in the study who stopped smoking reported they were not more stressed than women who continued to smoke.

In conclusion, the authors write that their "results are of considerable public health importance.

"The data suggest that the adverse effects of smoking on these late pregnancy outcomes may be largely reversible if smoking is ceased early in pregnancy, offering an important incentive for pregnant women who smoke to become smoke-free early in pregnancy."