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

<p><strong><span>Study question:</span></strong><span> Do the mothers of twins and singletons differ regarding post-partum and old-age mortality?</span></p> <p><span>&nbsp;</span><strong><span>Summary answer:</span></strong><span> M</span><span>others 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^thlifespan percentile.</span></p> <p><strong><span>What is known already:</span></strong><span> 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.</span></p> <p><strong><span>Study design, size, duration:</span></strong><span> 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 &ndash; 1 884 mothers of twins and </span><span>19 747 </span><span>&ndash; 36 690 mothers of singletons.</span></p> <p><strong><span>Participants/materials, setting, methods:</span></strong><span> 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&nbsp;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.</span></p> <p><strong><span>Main results and the role of chance:</span></strong><span> 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% (</span><span>449</span><span>/</span><span>122 750)</span><span> for single gestations (OR = 2.05, 95% CI = 1.21 &ndash; 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.</span></p> <p><strong><span>Limitations, reasons for caution: </span></strong><span>Relatively low number of individuals (22 802) with known age at death due to discontinuation of the register after 1943.</span></p>