Changes in the comprehensive unassisted pregnancy rate as a possible marker of declining human fecundity

Abstract

Recent decades have seen declining total fertility rates (TFR) globally, alongside increased use of assisted reproductive technology (ART). However, TFR includes ART births and excludes induced abortions, complicating assessments of population fecundity. Here, we examine trends in cohort total fertility rates (cTFR), induced abortions, and ART use through a nationwide cohort study of 1,648,971 pregnancies, including abortions, among Danish women aged 15-45 years born between 1958 and 1999. A new index, the Comprehensive Unassisted Pregnancy Rate (live births and induced abortions, excluding ART births), was developed. Our findings reveal a decline in unassisted pregnancy rates for women born after 1961, with an accelerated decline for those born after 1970. In contrast, cTFR increased for women born between 1958 and 1970 before decreasing, driven by trends in induced abortions and ART births. These differences highlight a disconnect between cTFR and fecundity measures. The declining unassisted pregnancy rates, reduced abortions, and increased ART demand raise concerns about population fecundity. Understanding these trends' biological and socioeconomic drivers requires large-scale, transdisciplinary studies of representative populations. Our results emphasize the need for alternative measures, like the Comprehensive Unassisted Pregnancy Rate, to assess reproductive health and fertility trends accurately.

Keywords: Assisted reproductive techniques; Contraceptives; Fecundity; Fertility; Induced abortions; Trends.

Fig. 1

Cohort total fertility rates (green) and Comprehensive Unassisted Pregnancy Rate (red) of Danish birth cohorts. The X-axis represents the birth cohorts from 1958 to 1983. The Y-axis represents the cohort total fertility rate (cTFR) and Comprehensive Unassisted Pregnancy Rate (CUPR), ranging from 0.0 to 2.5. Two plotted trend lines further indicate the rates. Note that the measures show opposite trends from 1961 to 1970, with a gradual shift in their difference from 0.53 for women born in 1958 to 0.31 for women born in 1983.

Fig. 2

Cumulative birth rates across birth cohorts of women born between 1958 and 1999. The X-axis represents women’s ages from 15 to 45 years. The Y-axis represents the cumulative birth rate per woman, ranging from 0.0 to 2.5. Multiple cohort lines ranging from the 1958 to the 1999 birth cohort are plotted on the figure. Each line indicates the respective cohort’s cumulative birth rate, starting from the lowest values at age 15 and peaking before age 45.

Fig. 3

Comprehensive Unassisted Pregnancy Rates (CUPR) across birth cohorts of women born between 1958 and 1999. The X-axis represents women’s ages from 15 to 45 years. The Y-axis represents the cumulative CUPR, ranging from 0.0 to below 2.5. Multiple cohort lines ranging from the 1958 to the 1999 birth cohort are plotted on the figure. Each line indicates the respective cohort’s CUPR, starting from the lowest values at age 15 and peaking before age 45.