Fathers who smoke may be putting their unborn children at risk of congenital heart defects (CHDs), according to research reported by scientists at Xiangya School of Public Health, Central South University, in Changsha, China. The research, carried out as a meta-analysis of data from 125 studies involving more than 100,000 children with CHDs, showed that maternal active smoking, maternal passive smoking, and paternal smoking all increased the risk of CHD in offspring. “Fathers-to-be-should quit smoking,” commented lead study author Jiabi Qin, PhD, who heads the team at Central South University. “Fathers are a large source of secondhand smoke for pregnant women, which appears to be even more harmful to unborn children than women smoking themselves.”

The researchers describe their study and results in the European Journal of Preventive Cardiology, in a paper titled, “Parental smoking and the risk of congenital heart defects in offspring: An updated meta-analysis of observational studies.”

Congenital heart defects affect approximately 8% of live births worldwide, and are a leading cause of death in newborns, the authors wrote. Prognosis and quality of life are improving thanks to innovative treatments, including surgeries, but the effects on affected children are nevertheless lifelong. Smoking is known to pose a risk to normal fetal development, Qin noted, and there is increasing interest in the potential link between parental smoking and heart defects in their offspring. “Smoking is teratogenic, meaning it can cause developmental malformations,” Qin said. “The association between prospective parents smoking and the risk of congenital heart defects has attracted more and more attention with the increasing number of smokers of childbearing age.”

However, the authors pointed out, while there have been many studies assessing the risk of CHDs associated with maternal passive smoking and paternal active smoking, the results have been inconsistent. Moreover, they pointed out, reviews of studies investigating the relationship between smoking and CHDs in offspring have focused primarily on maternal active smoking, “… and considered neither the relationship between maternal passive smoking and CHDs nor the relationship between paternal smoking and CHDs … In fact, maternal passive smoking and paternal smoking are more common than maternal active smoking.” Also, prior reviews haven’t considered the exposure time of smoking. “The critical stage of fetal heart development occurs in the early stages of pregnancy. Therefore, it is very important to focus on the association between parental smoking before pregnancy or in early pregnancy and CHDs, which may help to explain causality,” Qin’s team stated.

To try and provide a more comprehensive evaluation of the effects of smoking on CHD in offspring, the team designed an updated meta-analysis of observational studies to assess the link between maternal active and passive smoking, as well as paternal active smoking, on different types of CHDs. They identified 125 eligible studies published between 1971 and 2018, involving 137,574 cases of CHDs and 8.8 million prospective parents.

The results of their analyses indicated that, when compared with no exposure to cigarette smoke, all three types of parental smoking were associated with an increased risk for CHDs in offspring. “Our results suggested maternal passive smoking and paternal smoking were also significantly associated with risk of fetal CHDs, which has not been confirmed by previous meta-analyses,” the team wrote.

The greatest risk of CHD risk was associated with maternal passive smoking. “Findings from the present meta-analysis indicated that parental smoking was significantly associated with risk of CHDs in offspring, with an increased risk of 25% for maternal active smoking, 124% for maternal passive smoking, and 74% for paternal active smoking, compared with those with no smoking,” the authors reported. Interestingly, the link between CHD risk and all types of parental smoking was higher when the analysis was constrained to Asian populations.

When looking at different types of congenital heart defects, the researchers’ analyses also indicated that maternal smoking was associated with a 27% increased risk of atrial septal defect, and a 43% increased risk of right ventricular outflow tract. “… our review did not find statistically significant differences for risks of other CHD subtypes between comparison groups,” the researchers wrote. They were also unable to assess the relationship between either maternal passive smoking, or paternal active smoking and CHD subtypes because there were too few studies on each type of CHD.

The scientists suggested that their review represents the most up-to-date on the topic, and its large sample size provides more precise and reliable risk estimates than prior studies. It is also the first review to investigate the effects of smoking at different stages of pregnancy on the risk of CHDs. These data indicated that a woman’s exposure to someone else’s cigarette smoke even prior to pregnancy put her unborn child at risk of developing a CHD. In contrast, while the offspring of women who smoked during pregnancy were at an increased risk CHD, maternal active smoking before pregnancy didn’t’ affect the CHD risk.

“Women should stop smoking before trying to become pregnant to ensure they are smoke-free when they conceive,” said Qin. “Staying away from people who are smoking is also important. Employers can help by ensuring that workplaces are smoke-free. “Doctors and primary healthcare professionals need to do more to publicize and educate prospective parents about the potential hazards of smoking for their unborn child.”

The team noted that the exact mechanism involved in the association between parental smoking and CHDs in their offspring isn’t clear and will need further research. Nevertheless, they pointed out, some prior studies have indicated that maternal passive smoking has more harmful factors than maternal active smoking, “which was basically consistent with this study,” the team commented. “However, the mechanism of association between parental smoking and CHDs risk still needs to be further explored.”

The researchers acknowledged that their study does have several limitations. Nevertheless, they concluded, “ … our study indicated that maternal active and passive smoking, and paternal active smoking, are associated with higher risk of CHD … preventing parental smoking during peri-pregnancy is a priority for CHDs prevention.”