Exposure to secondhand smoke during pregnancy increases by about 2.3 times the risk of placental abruption, a condition in which part of the placenta separates from the uterine wall before childbirth, according to a study conducted by a team from Tohoku University.

The risk was found to grow by around 2.2 times for pregnant women who smoke, the team said. In both cases, the risk increased with the frequency of smoking or exposure.

Placental abruption is said to occur in 0.4% to 1% of pregnant women, and can fatally affect the health of the mother and the fetus. Other factors believed to cause placental abruption include maternal hypertensive disorder, high maternal age and abdominal impact, but the exact causal link is unknown.

The research group, led by Hirotaka Hamada, a lecturer at Tohoku University Hospital, analyzed data from a nationwide survey on children's health and their environments to study the relationship between placental abruption and both smoking and secondhand smoke in about 82,000 pregnant women.

It found that pregnant women exposed to secondhand smoke for four to seven days a week and at least an hour a day had 2.34 times the risk of suffering placental abruption compared with those who were not. The risk was 2.21 times higher among pregnant women who smoked at least 11 cigarettes per day compared with nonsmokers.

Of the pregnant women who actually experienced placental abruption, about 2.8% are believed to have suffered the condition due to maternal smoking and about 3% due to secondhand smoke exposure.

Toxic substances in tobacco have been found to reduce blood flow to the placenta, according to the research group.

"It is necessary to strengthen policies and educational activities to reduce secondhand smoke exposure in public places and homes," Hamada said.