Marriage still protects pregnancy.

Objective: To assess the risk factors and outcome of pregnancy outside marriage in the 1990s, in conditions of a high percentage of extramarital pregnancies and high standard maternity care, used by the entire pregnant population.

Design: Hospital-based cohort study.

Setting: A university-teaching hospital in Finland.

Population: The 25,373 singleton pregnancies of known marital and cohabiting status.

Methods: Odds ratios (ORs) with 95% confidence intervals were calculated to estimate the effect of extramarital childbearing on pregnancy outcome. Multiple logistic regression analyses were conducted to control for confounding maternal risk factors.

Main Outcome Measures: Small-for-gestational age (SGA) infants, preterm birth (less than 37 completed weeks), low birthweight (LBW; under 2500 g).

Results: Of the study population, 67.5% were married and 32.5% were unmarried; 24.2% of all mothers were cohabiting. Unmarried status was strongly associated with social disadvantage and particular risk factors, specifically unemployment, smoking and previous pregnancy terminations, which in turn had an impact on obstetric outcome. There were significantly more SGA infants among unmarried mothers (P < 0.001), with an absolute difference of 45%; more preterm deliveries (P= 0.001), with an absolute difference of 17.5%; and more LBW infants (P < 0.001), with an absolute difference of 26%. The differences in adverse pregnancy outcomes between study groups (i) all unmarried women, (ii) cohabiting women and (iii) single women, remained significant after multivariate analysis at adjusted ORs of 1.11, 1.11 and 1.07 for SGA, 1.17, 1.15 and 1.21 for LBW and 1.15, 1.15 and 1.29 for the preterm births, respectively.

Conclusion: Even in the 1990s when cohabitation was already common, pregnancy outside marriage was associated with an overall 20% increase of adverse outcomes, and free maternity care did not overcome the difference.