Even if a woman quits smoking once she finds out she is pregnant, she can give birth to a baby weighing the same as a child born to a non-smoker, a British expert told a European fertility meeting Wednesday.
"Once you find you're pregnant, its not too late to do something about your smoking," Nick Macklon, a professor of obstetrics and gynaecology at the University of Southampton, told AFP.
"If you stop smoking, you can have a baby with the same birth weight as if you'd never smoked," he explained, adding birth weight was an important predictor of long-term health.
But "if you find you're pregnant and you continue smoking, that will have a major effect on the birth weight of your child," he warned.
Macklon's findings, presented at the annual conference of the European Society of Human Reproduction and Embryology (ESHRE) in Stockholm, are based on a study of more than 50,000 pregnant women in Southampton between 2002 and 2010.
Researchers have known for many years that smoking is detrimental to foetal health, but Macklon said this is the largest study to date showing how stopping smoking improves pregnancy outcomes.
Adjusted for gestational age, maternal age, BMI (body mass index) and socio-economic class, the probe found that those babies whose mothers had stopped smoking around the time of getting pregnant or as soon the pregnancy was confirmed had a baby with a significantly higher birth weight, the researchers said in a statement.
The study showed that women who quit smoking once they discovered they were pregnant had babies that weighed an average of 300 grammes (10.6 ounces) more than babies born to women who smoked throughout their pregnancy.
"I think that's very encouraging news," said Macklon, who also works at Southampton's Complete fertility centre.
"While many couples will try to prepare themselves for pregnancy and adjust their lifestyle, still the majority of pregnancies are unplanned... What we are able to say is 'even if you haven't planned it, it's still not too late to do something about it'."
The researchers started following the women at the time of the first pregnancy-related medical visit, or about seven or eight weeks into term.
Outwardly, this might suggest that women can smoke almost two months into their pregnancy and still give birth to normal-weight babies.
But Macklon stressed the negative effects of tobacco on an unborn child were far greater than only birth weight.
Foetal abnormalities and cleft lip were among the effects that could be caused by a woman smoking before she realises she is expecting -- and even before the baby is conceived.
"The message ought to be 'stop smoking before you conceive,' but in the real world, many people fall pregnant unplanned, and therefore there is still a positive a message for them," he added.