The role of biological fertility in predicting family size.
BACKGROUND
It is plausible that a couple's ability to achieve the desired number of children is limited by biological fertility, especially if childbearing is postponed. Family size has declined and semen quality may have deteriorated in much of Europe, although studies have found an increase rather than a decrease in couple fertility.


METHODS
Using four high-quality European datasets, we took the reported time to pregnancy (TTP) as the predictor variable; births reported as following contraceptive failure were an additional category. The outcome variable was final or near-final family size. Potential confounders were maternal age when unprotected sex began prior to the first birth, and maternal smoking. Desired family size was available in only one of the datasets.


RESULTS
Couples with a TTP of at least 12 months tended to have smaller families, with odds ratios for the risk of not having a second child approximately 1.8, and for the risk of not having a third child approximately 1.6. Below 12 months no association was observed. Findings were generally consistent across datasets. There was also a more than 2-fold risk of not achieving the desired family size if TTP was 12 months or more for the first child.


CONCLUSIONS
Within the limits of the available data quality, family size appears to be predicted by biological fertility, even after adjustment for maternal age, if the woman was at least 20 years old when the couple's first attempt at conception started. The contribution of behavioural factors to this result also needs to be investigated.
Department of Epidemiology and Public Health, Imperial College, St Mary's Campus, Norfolk Place, London W2 1PG, UK. m.joffe@imperial.ac.uk
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D003268:Contraception Behavior; D005191:Family Characteristics; D005260:Female; D005298:Fertility / Q000502:physiology*; D006801:Humans; D008297:Male; D008423:Maternal Age; D015233:Models, Statistical; D011247:Pregnancy; D012907:Smoking / Q000009:adverse effects