While it's been known for some time that smoking may be damaging to a woman's reproductive health,1,2 a new study finds that so-called 'side-stream' smoke can be just as destructive.3
The findings were published in the journal Human Reproduction.3
Passive Smoke May be Just as Risky
Side-stream smoke is a form of passive smoking—that which is given off by a smoldering cigarette, which the researchers say contains the most toxic elements. Passive smoke includes both side-stream smoke and exhaled smoke.
"There is a strong association between cigarette smoking and reduced fertility, and earlier mean age of menopause, suggesting that smoking impairs [egg] function [and] viability, and depletes ovarian reserves," wrote doctors in the new study, but adding that "although cigarette smoke is a recognized reproductive hazard, the impact of passive or sidestream smoke exposure on female fertility is unknown."
To clarify the risk, the research team from McMaster University in Ontario and the Center for Reproductive Care, an Ontario fertility clinic, initiated a study of 225 women undergoing in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) over a 1-year period. The investigators assessed each patient's embryo quality, as well as implantation and pregnancy rates during that time. Beforehand, the women in the study were grouped according to whether they were non-smokers, smokers, or those exposed to side-stream smoke; that is, women who lived with a partner who regularly smoked, but who didn’t directly smoke themselves. Former smokers were grouped with non-smokers since previous studies have shown that the fertility of former smokers is similar to that of non-smokers, wrote the investigators.
Each woman who underwent IVF used typical protocols, and had their eggs retrieved after injections of human chorionic gonadotropin (hCG), a drug used to induce ovulation. The rest underwent ICSI four to six hours after egg retrieval. This is a procedure in which one sperm is injected into an egg for fertilization and is typically prescribed in cases in which there is male factor infertility.4
The investigators then compared the number of eggs retrieved, the fertilization rate, number of embryos transferred to the uterus for pregnancy, the number of embryos frozen, embryo quality, and implantation and pregnancy rates between the three groups, among other things.
Smoking Groups: More Negative Results
There were no differences in the ages of the patients, meaning age had no effect on IVF or ICSI outcome in this study. The number of eggs collected and embryos transferred were also similar.
But, while fertilization rates and embryo quality were similar in all three groups, there was a significant difference in the pregnancy rate per embryo between the groups, with those in the smoking and side-stream smoking group having the least success, the study team reported. The pregnancy rate in the mainstream smoking group was about 19.4%. It was 20% in the side-stream smoking group. But that compares to about half of the non-smoking women who got pregnant, the researchers found.
There were more telling findings. "When it came to implantation rates, which we calculated as the number of fetal sacs with a positive heartbeat divided by the total number of embryos transferred, we found that while non-smokers achieved a 25% implantation rate, both smokers and side-stream smokers managed only about 12%," said Michael Neal, the study's lead researcher and a PhD candidate at McMaster University.
A Word of Caution
Evidence from this study is prompting the research team to warn all of their patients of the potential hazards, Neal said. He and his colleagues point out that 10 times the level of contaminants known as polycyclic aromatic hydrocarbons from the tip of a smoldering cigarette were found in those exposed to second-hand smoke compared to women who were mainstream smokers. Thus, "we propose that side-stream cigarette smoke exposure is as hazardous as that of mainstream smoke," they wrote.
Still, Warren Foster, PhD, an associate professor of Obstetrics and Gynecology at McMaster University and the study's senior researcher says this was a retrospective analysis relying on self-reported smoking habits. As such, the findings should be confirmed in a prospective study with more objective measures of cigarette smoke exposure, which could also answer questions about whether higher levels of smoke have a more detrimental impact.
But Foster remains cautious, nonetheless. "Although we do need a prospective confirmatory study, the findings from our study already warrant a warning to women to reduce or, if possible, prevent exposure to cigarette smoking, especially if they are trying to conceive," he said.
Foster and his colleagues are hoping to launch a prospective study soon.
In the meantime, they are hoping to determine why there was no significant difference in embryo quality between the three groups of women in this study, but at the same time, implantation success was still lower in the women in the two smoking groups.
"This was the most striking finding from our study," said Neal.
1. Wainer R. Smoking and ovarian fertility [Translated from French]. Gynecol Obstet Fertil 2001 Dec;29(12):881-7.
2. Van Voorhis BJ, Dawson JD, Stovall DW, Sparks AE, Syrop CH. The effects of smoking on ovarian function and fertility during assisted reproduction cycles. Obstet Gynecol 1996 Nov;88(5):785-91.
3. Neal MS, Hughes EG, Holloway AC, Foster WG. Sidestream smoking is equally as damaging as mainstream smoking on IVF outcomes. Hum Reprod doi:10.1093/humrep/dei080.
4. Al-Hasani S, Ludwig M, Palermo I et al. Intracytoplasmic sperm injection of round and elongated spermatids from azoospermic patients: results and review. Hum Reprod 1999 Sep;14 Suppl 1:97-107.
John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include overseeing health news coverage for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.
