Working paper no. 2009-03
Infant mortality at birth may help illuminate the usefulness of season of birth as an indicator of early life exposures. We obtained data for infant mortality rates (IMR) at the county (municipio) level during the late 1920s-early 1940s in Puerto Rico using historical records and linked IMR with individual birth year and place using the PREHCO (Puerto Rican Elderly: Health Conditions) study. We classified PREHCO respondents into two groups according to high or low infectious disease load, corresponding to lower or higher proportion endogenous mortality in the year respondents were born. We estimated the effects of IMR (using continuous, logit, quartile, Box-Cox transformations) and season of birth on adult heart disease and diabetes for all respondents and then by subgroups, controlling for age, gender, obesity, respondent’s educational level, adult behavior (smoking, exercise) and other early life exposures (childhood health, knee height, childhood SES). Findings: (1) no significant associations between IMR and heart disease or diabetes but significant associations between high IMR and low knee height, low education, older age and no rigorous exercise as an adult; (2) stronger effects of season of birth on adult health among respondents born in years with lower infectious disease loads; (3) strong effects of season of birth even after controlling for other childhood and adult factors. Conclusions:(1) effects of poor environmental conditions during the first year of life on adult health may be mediated by other life course factors; (2)season of birth is a useful indicator of early life exposures under restricted conditions; (3) in this population of older Puerto Rican adults, season of birth provides (weak) evidence in support of the importance of in utero (endogenous) causes affecting adult heart disease and diabetes.