Time-trends in secondary sex ratio of firstborns and polychlorinated biphenyls in Norway for the time period 1967 - 2018

Abstract

Background: The secondary sex ratio (SSR), the ratio of males to females at birth, has been of much interest in epidemiologic research, as the ratio may be affected by endogenous and exogenous factors. There have been several claims of a declining SSR from the second half of the 20th century, and environmental contaminants, especially persistent organic pollutants (POPs) including polychlorinated biphenyls (PCBs) have been suspected as a cause due to the chemical’s endocrine disrupting abilities. Objectives: The overall aim is to explore if there are any similarities in time trends of SSR in firstborns in Norway and predicted maternal PCB serum concentrations in the time period 1967-2018. Specifically, we used population-level data for SSR, and predicted exposure trends for PCB-153 as a proxy for PCBs during this time period that included the period when PCB exposure was at their highest levels in humans in Norway. Materials and method: The data used in the thesis is gathered from the Medical Birth Registry of Norway (MBRN) for the time period 1967-2018. We included firstborn singletons after week 21 of gestation and had a total study population of 1,233,767. We stratified our data by maternal age by years 20-24, 25-29, 30-35 and 35-39. Maternal serum concentrations of PCB-153 was predicted using the CoZMoMAN model. Results: The results demonstrated no declining trend in SSR throughout the time period 1967 to 2018, as linear regression lines were flat and coefficients for change per birth year was close to zero for all maternal age groups. SSR did not appear to deviate from the overall SSR around the time period of peak PCB concentrations, more than yearly fluctuations in any other periods. Although the total number of births remained relatively unchanged, the maternal age composition has changed drastically throughout the study period. Conclusion: Our findings indicate no change in SSR for firstborns in Norway in the study period 1967-2018. Further, there was no indication of any alterations in SSR on a population level in Norway in the years around the peak exposures of PCBs.