Some of the severe impacts of tobacco on newborns can be reduced if the mother quits smoking by the 15th week of pregnancy, according to a study published Friday in the British Medical Journal.
A pregnant woman who smokes exposes her foetus to the well-established risk of being born prematurely and with a low birthweight.
Doctors in Australia and New Zealand monitored 2,500 pregnant women, asking them about their smoking habits at the 15th week of pregnancy and then surveying the outcome at birth.
Eighty percent of the women were non-smokers; 10 percent had stopped smoking; the remaining 10 percent were current smokers at the time of the first phase of the investigation.
One in 10 of the "current smokers" had babies who were born ahead of term, and 17 percent of their infants had a lower birthweight.
But there was no difference between non-smokers and those who had stopped smoking by the 15th week of pregnancy.
The rates in both groups were four percent for pre-term delivery and 10 percent for a lower birthweight respectively.
"These severe adverse effects of smoking may be reversible if smoking is stopped early in pregnancy," said the paper, led by Lesley McCowan, of the Department of Obstetrics and Gynaecology at the University of Auckand, New Zealand.
Smoking during pregnancy also carries many other risks, including miscarriage, ectopic pregnancy, stillbirth and neo-natal death, according to previous research.