Introduction Socioeconomic conditions are rarely considered in the epidemiology of subfertility. This study determined the effect of current and past socioeconomic conditions on the occurrence of sub-fertility in women with a successful pregnancy.

Methods Women were recruited in 2005/6 for Geração XXI, a Portuguese birth cohort. Personal interviews were conducted to collect socio-demographic and pregnancy-related data. Life-time subfertility was self-reported and defined as trying to conceive for more than 1 year with no success. Education, income, working condition and marital status were proxy indicators of social class. The childhood circumstances of the women were accessed by parents' education and amenities at age 12. The analysis considered 7916 mothers. Logistic regression models were fitted to estimate the odds (OR) of being sub-fertile according socioeconomic circumstances, stratified by number of previous pregnancies (1 vs >1) and age (<30 vs ≥30 years).

Results Sub-fertility was reported by 9.1% of the women (n=719). Prevalence was 8.7% among primigravidae and 9.5% in multigravidae (84% of the multigravidae were multiparous). Almost 2/3 sought medical help. Among young primigravidae, sub-fertility decreased with education (>12 vs ≤6 years: OR=0.20 95% CI 0.11 to 0.39), and was lower among single women (OR=0.27 95% CI 0.12 to 0.59) and smokers (OR=0.60; 95% CI 0.40 to 0.94). Only education showed a significant association with sub-fertility in older primigravidae. Within multigravidae higher education levels increased the odds. Maternal childhood conditions were no longer significant after adjustment for current social circumstances.

Conclusions Age and the number of previous births modified the effect of social conditions on sub-fertility which was stronger among younger and primigravidae women.