Image caption Fathers who smoke should be aware of the danger they pose to their unborn child, experts say

Fathers-to-be should stop smoking to protect their unborn child from the risk of stillbirth or birth defects, scientists say.

University of Nottingham researchers found that pregnant women exposed to smoke at work or home increased their risk of stillbirth by 23% and of having a baby with defects by 13%.

They looked at 19 previous studies from around the world.

A UK expert said it was "vital" women knew the risks of second-hand smoke.

The studies used to pull this research together were carried out in North America, South America, Asia and Europe.

All the studies focused on pregnant women who did not smoke themselves but were passive smokers due to their proximity to a partner who smoked or work colleagues who smoked.

The combined data from the studies suggests that being exposed to more than 10 cigarettes a day is enough for the risks to be increased.

It is very important that men quit smoking before trying for a baby. Dr Jo Leonardi-Bee, University of Nottingham

However, the University of Nottingham study did not find an increased risk of miscarriage or newborn death from second-hand smoke - only an increased risk of still birth and birth defects.

The results did not point to a link with any specific congenital birth defect.

Impact on sperm development

The researchers say fathers who smoke should be more aware of the danger they pose to their unborn child.

Previous research has shown that women who smoke during their pregnancy create serious health risks for their unborn baby, including low birth weight, premature birth and a range of serious birth defects such as cleft palate, club foot and heart problems.

Dr Jo Leonardi-Bee, lead researcher of the study and associate professor in medical statistics at the University of Nottingham, said they still did not know when the effects of the second-hand smoke begin.

"What we still don't know is whether it is the effect of sidestream smoke that the woman inhales that increases these particular risks or whether it is the direct effect of mainstream smoke that the father inhales during smoking that affects sperm development, or possibly both.

"More research is needed into this issue although we already know that smoking does have an impact on sperm development, so it is very important that men quit smoking before trying for a baby."

Dr Leonardi-Bee added: "The risks are related to the amount of cigarettes that are smoked so it is therefore very important for men to cut down.

"Ultimately though, in the interests of their partner and their unborn child, the best option would be to give up completely."

Andrew Shennan, professor of obstetrics at St. Thomas' Hospital in London and spokesperson for baby charity Tommy's, said: "It is vital that women are made aware of the possible risks associated with second-hand smoke and alert those around them of the impact it could potentially have on the health of their unborn baby.

"The chemicals in cigarettes are known to significantly increase the risk of serious pregnancy complications."