Season of birth in Puerto Rico: Rethinking changes in patterns using 90 years of births occurring under US occupation.

This article examines birth seasonality in Puerto Rico and explores whether patterns shift during periods of economic transformation. We compiled monthly birth counts for approximately 5.64 million births occurring from 1935 to 2024. Data sources include reports to U.S. agencies (1935-1966), a prior publication (1967-1971), and the Puerto Rico Vital Statistics System (1972-2024). To assess the temporal concentration, we calculated a seasonal index of births and modeled 5-year patterns with LOESS curves. We identified four distinct seasonal patterns. From 1943 to 1949, births peaked in April - May. Between 1950 and 1954, this peak declined as a new one emerged from August to October. Beginning in 1955, a third pattern appeared, with fewer births early in the year and a peak in the second half. This pattern remained stable until about 2010. After 2010, we observe (1) a narrowing of early-year minimums, (2) increased within-period variability, and (3) a decline in December, though (4) the August to November peak persists. These changes align with economic transitions that occurred under US occupation, bringing forth the need to reexamine the hypothesis of cultural or media influence in future studies. Given observed patterns, future research should explore the role that economic shifts may have in shaping birth seasonality in Puerto Rico.