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

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.