The Missing Siblings of Infants Born Preterm

Parents of very or extremely low birth weight infants have fewer subsequent children after preterm birth. Whether this applies to parents of less preterm infants is unknown. In this nationwide cohort study, we identified all 230 308 traceable (>99%) singletons (9983 preterm, 4.3%) live born in Finland between January 1, 1987, and September 30, 1990, and their parents. Quantitative contribution of gestational age of child to the birth of parental subsequent children was assessed by multivariate Cox regression models, stratifying by the number of previous children. The impact of gestational age on sibling count was estimated at individual and population level. Mothers of extremely preterm (23-27 completed weeks) infants were, compared with mothers of term infants (39-41 weeks), less likely to have a subsequent live-born child (adjusted hazard ratio [HR]: 0.74; 95% confidence interval: 0.63-0.86). Corresponding HRs and confidence intervals were as follows: 28 to 31 weeks: 0.72 (0.65-0.80), 32 to 33 weeks: 0.82 (0.74-0.90), and 34 to 36 weeks: 0.90 (0.87-0.93). These HRs were consistent with those of fathers and couples. The cohort included 8002 firstborn preterm children, of whom 356 (4.4%) died in infancy. The 8002 children had a total of 13 826 subsequent siblings (1138 less than expected); per 1000 preterm births, this translates to the death of 44 preterm infants and 142 missing subsequent siblings. Families with a preterm singleton child have fewer subsequent children. In a high-income country, the main population effect of preterm birth is caused by these "missing siblings," whose number exceeds the number of those preterm infants who die.