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Smoking, delayed conception and infertility


The number of young women who smoke is increasing. This meta-analysis aims to determine whether there is an association between smoking and risk of infertility in women of reproductive age.

Message

Women who smoke have an increased risk of about 40% of delaying conception by over a year.

Reference

C Augood et al. Smoking and female infertility: a systematic review and meta-analysis. Human Reproduction 1998 13: 1532-1539.

Search

MEDLINE and Embase databases were searched from 1966 and 1974 respectively to 1997. Bibliographies of identified studies were then reviewed. To be included studies had to examine active cigarette smoking by women of reproductive age.

Twelve studies (eight cohort and four case-control) were identified.

Cohort studies: Participant numbers in each study ranged from 622 to 6,630, with a total of 20,059. All but one study was retrospective (the women were either pregnant or mothers). The majority defined infertility as taking more than twelve months to conceive. Seven of the eight studies adjusted for age.

Case-control studies: The outcomes measured in the case-control studies were primary infertility (trying to conceive for over a year), primary tubal infertility or secondary infertility (trying to conceive for over 18 months). Only one of the four studies adjusted for age. Infertility cases totalled 3,941.

All studies adjusted for a variety of other factors (e.g. intake of alcohol and coffee, body mass index), but not consistently so. Former and non-smokers were analysed together.

Results

In the cohort studies, the risk of delaying conception by over a year in smoking women was 42% compared with non-smoking women (odds ratio 1.42, 95% relative risk 1.27 to 1.58).

In the case-control studies there was an increased risk of infertility in smoking women compared with non-smoking women (odds ratio 2.27, 95% confidence interval 1.28 to 4.02).

Comment

It is not possible to quantify the increased risk of infertility from the case-control studies. There were only four studies with a small number of infertility cases; three different outcomes were measured and only one adjusted for age. Of the cohort studies, all but one was retrospective and they did not adjust consistently for other health-related behaviours of the women. It is important to bear in mind that smokers may have different lifestyles which also affect conception. It was also not possible to examine the relationship between number of cigarettes smoked and the risk of delayed conception. Nevertheless, this is a useful analysis, which highlights another danger of smoking.