Mothers of twins had higher old-age survival than mothers of singletons in Estonian 19th-century birth cohorts

Study question: Do the mothers of twins and singletons differ regarding post-partum and old-age mortality?

 Summary answer: Mothers of twins had twice as high post-partum mortality as mothers of singletons; survival of twinners was higher than survival of the mothers of singletons after the 67^th lifespan percentile.

What is known already: Twinning is typically associated with higher post-partum maternal mortality. The evidence about whether twinning incurs long-term survival costs of reproduction or is a trait pertinent to long-lived women is scarce and contradictory.

Study design, size, duration: The study is based on the data of the Estonian Family Register (operating from 1926-43) and involves 5 565 mothers of twins and 119 613 mothers of singletons born between 1850-99. The subset for comparing maternal lifespans included 1 703 – 1 884 mothers of twins and 19 747 – 36 690 mothers of singletons.

Participants/materials, setting, methods: Post-partum maternal mortality was analysed in the whole sample (including mothers of a single child) by logistic regression. Most of the analyses were performed in samples where each mother of twins was matched against mothers of singletons based on parity, urban versus rural origin, whether their lifespan was known, date of birth and age at first birth. Quantile regression was used to analyse age-dependent variations in maternal mortality rates. Lifespans were compared in linear mixed models. All models were adjusted for relevant biodemographic covariates.

Main results and the role of chance: The twinning rate in the whole sample was 4.4%. During the year after giving birth, maternal mortality for multiple gestations was 0.75% (17/2 273) and 0.37% (449/122 750) for single gestations (OR = 2.05, 95% CI = 1.21 – 3.23). The association between twinning and post-natal maternal mortality remained significant in a model controlling for parity and age of first and last birth. The life spans of the mothers of twins and singletons did not differ in matched samples. Past the 67^th lifespan percentile, the odds of survival were significantly higher for mothers of twins than mothers of singletons, as indicated by non-overlapping 95% confidence intervals.

Limitations, reasons for caution: Relatively low number of individuals (22 802) with known age at death due to discontinuation of the register after 1943.