Longitudinal trends in fertility in women of advanced maternal age in the United States and Sweden from 1935-2018 and comparison to maternal mortality ratios.

Advanced maternal age (AMA, >35 years at delivery) confers maternal and foetal risks, particularly with age >45 years and nulliparity, but longitudinal comparative data on age- and parity-specific AMA fertility is lacking. We used the Human Fertility Database (HFD), a publicly available, international database, to analyse fertility in US and Swedish women aged 35-54 from 1935 to 2018. Age-specific fertility rates (ASFR), total birth counts, and proportion of AMA births were evaluated across maternal age, parity, and time, and compared to maternal mortality rates during the same time. In the US, total AMA births nadired in the 1970s, and have risen since. Until 1980, most AMA births were to women completing parity 5 or higher; since then, most have been to low parity women. While ASFR in 35 to 39 year olds was highest in 2015, ASFR in women 40-44 and 45-49 were highest in 1935, though they have been rising recently, especially in low-parity women. While the same AMA fertility trends were seen in the US and Sweden from 1970-2018, maternal mortality rates have risen in the US despite remaining low in Sweden. Although AMA is known to contribute to maternal mortality, this discrepancy merits further consideration.