Does delayed childbearing contribute to rates of multiple birth?
Pregnancy in women aged 35 year increased nine-fold between 1972 and 2012. As this shift occurred, plural birth also increased, prompting researchers to look for connections between the two trends. Results of a study in Obstetrics and Gynecologyhelp quantify the contribution of delayed childbearing to the national plural birth excess.
Using data reported by the National Center for Health Statistics, the authors estimated the contribution of delayed childbearing to the national plural birth excess through 2016. They compared the national birth cohort’s plural birth rates before use of assisted reproductive technology (ART) with those after ART emerged.
Before ART (1949 to 1966), the national birth cohort (n=71,570,717) consisted of 1.4 million twin and more than 19,000 higher-order plural births. Between 1971-2016, the era during which ART emerged, the national birth cohort (n=166,817,655), consisted of 4.3 million twin and nearly 175,000 higher-order plural births. White and black maternal race categories accounted for 99% or the total national birth complement between 1949 and 1966. Between 1971 and2016, the white and black race categories accounted for 89% of the annual total birth complement.
Relative risk estimates of unassisted plural births by maternal age before the ART era (1949-1966) were unimodal and race-dependent. For black women aged 35 to 39, risk of unassisted plural births was 2.75-fold higher than their 15- to 19-year-old counterparts (39.81/1,000 births to 14.48/1,000 births; 95% CI 2.67-2.83). In comparably aged white women, a 2.47-fold risk increment was found (28.76/1,000 births to 11.63/1,000 births; 95% CI 2.43-2.52).
The authors noted a rising trend of unassisted plural births during the latter era in both white and black women. White women had 255,964 (95% CI 134,746-375,581) unassisted plural births during this period versus 66,271 (95% CI 34,099-96,197) in black women. In 2016 alone, delayed childbearing accounted for 24% (95% CI 28-47%) of the national plural birth excess for white women and 38% (95% CI 28-47%) for black women.
The authors also included a simulated projection through 2025 (with the assumption that trends observed through 2016 will continue apace), and concluded that delayed childbearing could account for as much as 46% (95% CI 28-47%) of unassisted plural births for white women and as much as 40% (95% CI 30-53%) for black women.
The authors note a few strengths and limitations of their study. Among the identified strengths were the reliance on large birth cohorts and use of time intervals preceding and coinciding with the ART era. Limitations included absence of variables other than age and race, absence of family history, and the unfeasibility of correcting for coincident decrements in perinatal and maternal mortality resulting from healthcare improvements.
The researchers also noted that their 2025 projections assumed that current trends will persist through the next decade and could be erroneous. While the authors acknowledged that more in-depth research is necessary, their findings, they said, indicate that delayed pregnancy has a role in the plural birth excess and ob/gyns and their patients should be aware of the effects when discussing family planning decisions.