Fertility rates across generations in twins and singletons: A total population study in Finland

Study question: Does twin status and zygosity (monozygotic vs. dizygotic; same-sex vs. opposite-sex) predict fertility outcomes and intergenerational reproductive patterns compared with singletons? Summary answer: Among females, dizygotic twins had modestly higher completed fertility than singletons and monozygotic twins and were more likely to have a twin birth. Fertility did not differ meaningfully among males. These differences were restricted to the twin generation and did not persist in the next generation, indicating sex-specific and generation-specific effects rather than intergenerational transmission. What is known already: Dizygotic twinning is associated with heritable hyperovulation and higher natural fertility but less is known about whether being a twin or zygosity influences reproductive outcomes across generations. Study design, size, duration: A population-based longitudinal cohort study using part of the Finnish Twin Cohort and national population registers. Participants included monozygotic (MZ; N = 4,068), same-sex dizygotic (SSDZ; N = 8,890), opposite-sex dizygotic (OSDZ; N = 8,474) twins, and singleton controls (N = 1,193,404) born between 1945-1957 (total N =1,254,103; 49.1% female), their mothers, their children, and their grandchildren. Participants/materials, setting, methods: Fertility outcomes (number of biological children, age at first birth, childlessness, multiple births) were derived from Finnish population registers. Analyses followed a preregistered plan (https://osf.io/qbwv3) Main results and the role of chance: Differences in fertility between singletons and twins were modest and varied by sex and zygosity. Differences were observed generally in the mothers of twins and female twins themselves, with limited differences in the offspring of twins as compared to the offspring of singletons. Twins were slightly older at first birth, had fewer total biological offspring, but were more likely to have a twin birth. Dizygotic twins in particular differed from monozygotic twins and singletons. Limitations, reasons for caution: Findings are limited to individuals born in mid-20th-century Finland and thus generalizability to recent populations or non-Nordic contexts may be restricted. Further, analyses are observational, and causal inference is limited due to alternative motivation behind fertility rates like social or cultural reasons. Wider implications of the findings: These findings suggest that zygosity and sex interact to shape reproductive outcomes, offering insight into genetic and environmental contributions to fertility. They highlight the value of large twin cohorts for studying intergenerational reproductive trends and the representativeness of twins in population-based fertility research.