Delayed Childbearing as a Growing, Previously Unrecognized Contributor to the National Plural Birth Excess

OBJECTIVE: To establish the annual and cumulative multiyear contribution of delayed childbearing to the national plural birth excess. METHODS: Using publicly available national birth data reported by the National Center for Health Statistics, we estimated the contribution of delayed childbearing to the national plural birth excess through 2016. To this end, the observed as well as the maternal age-and race-adjusted national plural birth rates were assessed before (1949-1966; n=71,570,717) and during (1971-2016; n=166,817,655) the assisted reproduction era. Comparable estimates through 2025 relied on simulated projections. RESULTS: The relative risk estimates of unassisted plural births by maternal age before the assisted reproduction era (1949-1966) proved unimodal and race-dependent. The risk of unassisted plural births in 35- to 39-year-old black women proved 2.75-fold higher than that of 15- to 19-year-old counterparts (39.81/1,000 to 14.48/1,000; 95% CI 2.67-2.83). A 2.47-fold risk increment was noted for comparably aged white women (28.76/1,000 to 11.63/1,000; 95% CI 2.43-2.52). Similar age-dependent risk increments were obtained for twin and higher order births. An increasingly prominent delayed childbearing trend during the assisted reproduction era (1971-2016) gave rise to 255,964 (95% CI 134,746-375,581) and 66,271 (95% CI 34,099-96,197) unassisted plural births by white and black women, respectively. In 2016 alone, delayed childbearing accounted for 24% (95% CI 15-32%) and 38% (95% CI 28-47%) of the national plural birth excess for white and black women, respectively. By 2025, delayed childbearing could account for as much as 46% (95% CI 32-60%) and 40% (95% CI 30-53%) of the national plural birth excess for white and black women, respectively. CONCLUSION: Delayed childbearing, a growing contributor to the national plural birth excess previously solely ascribed to assisted reproduction, warrants greater consideration in future clinical, analytic, and policy deliberations as well as in individual family planning decisions.