(Reuters Health) - Would-be fathers may increase their partner’s risk of miscarriage by smoking during the pregnancy, or even during the time leading up to conception, a large study from China suggests.

Based on data for nearly 6 million pregnancies, researchers found that women whose partner smoked during the first few months of the pregnancy were 17 percent more likely to miscarry than women with nonsmoking partners.

Women whose partner quit smoking around the time of conception had an 18 percent lower risk of miscarriage than those whose smoking partner didn’t quit, the study team reports in the Journal of Epidemiology & Community Health.

“Although we have known for a long time that if the mother smokes there is an increased risk for adverse pregnancy outcomes, dads who smoke also influence the ‘success’ of the pregnancy,” Dr. Alison Holloway, who was not involved in the study, said in an email.

The study’s size is especially noteworthy, said Holloway, a professor of obstetrics and gynecology at McMaster University in Hamilton, Ontario.

The authors collected data from nearly 5.8 million non-smoking women aged 18-49 and their husbands in rural China. The couples were participating in a national free pre-pregnancy checkups project from 2010 to 2016. That service included three stages: a preconception health examination, an early-pregnancy follow-up within the first three months after conception, and a pregnancy outcome follow-up about a year after the early-pregnancy exam.

About 29 percent of the male partners were smokers, and the overall rate of miscarriages was about 2.5 percent.

When the male partner was a nonsmoker, however, the miscarriage rate was 2.38 percent, compared with 2.92 percent when the male partner smoked.

When the prospective father quit smoking before or just after conception, the miscarriage rate was 2.79 percent, compared with 3.35 percent when the father did not quit.

“The importance of tobacco control, specifically pertaining to paternal smoking, should be emphasized, and husbands should quit smoking when planning a pregnancy,” write the study’s authors, many of whom work for China’s National Research Institute for Family Planning in Beijing, and who did not respond to requests for comment.

While the authors identified an association between male partner smoking and miscarriage, the study doesn’t prove that one caused the other. It also did not examine the mechanisms by which paternal smoking might influence pregnancy loss.

There are at least two possible ways that a husband smoking might influence a miscarriage, Holloway said. A husband smoking might expose his wife to chemicals through secondhand and thirdhand smoke - that is, smoke deposits on clothes, furniture, carpets, etc. Alternatively, smoking might impact the quality of the father’s sperm, leading to genetic mutations that could in turn lead to pregnancy loss.

“Although they have addressed the impact of paternal smoking, you still can’t determine how dad’s smoking behavior impacts miscarriage risk,” she told Reuters Health.

The inability to demonstrate causality is one of the study’s main limitations, Dr. Zev Williams, Chief of the division of reproductive endocrinology and infertility at Columbia University Medical Center in New York City, said in an email. He also noted that the study was limited by its dependence on patient recollection and didn’t measure actual levels of many of the toxins produced by smoking.

Williams, who was not involved in the study, described pregnancy as “a precious but precarious thing.”

It is “very important to try to optimize all the factors that can help achieve a healthy pregnancy,” he noted. “While a lot of focus has been on the mother’s health, this study highlights the importance of her environment and the partner’s contribution to the health of the pregnancy.”

SOURCE: bit.ly/2MtnwC7 Journal of Epidemiology & Community Health, online June 11, 2018.