Time Trend of the Male Proportion at Birth in Brazil, 1979-2004

Gerusa Gibson,Luciana Scarlazzari Costa,Sergio Koifman

doi:10.3390/ijerph6082193

Abstract

Several studies suggest that exposure to environmental endocrine disrupters can reduce the number of male births, and therefore, the male proportion at birth (also denominated birth ratio or sex ratio at birth) can be used as a sentinel health indicator. This work aimed to analyze the trend of male births in Brazil, according to their distribution by states and capitals. An ecological descriptive time series was carried out using polynomial regression, showing a declining trend for male proportion at birth in Brazil (1979–1994), followed by an upward trend until 2004. A decline on the proportion of male births was observed in Brazil between 1979 and 1993, followed by a subsequent rise of this ratio between 1995 and 2004, wherein the mean proportion of male births in Brazil rose from 51.05 to 51.18, representing a relative increase of 0.25%. The states of São Paulo (Southwest region) and Acre (Western Amazon), as well as some capitals–Cuiabá (Western Region), Palmas (Amazon) and Rio Branco (Amazon)–showed increasing trends, which suggests the influence of socio-demographic changes. In contrast, a declining trend in the State of Ceará State (Northeast region), with a 0.35% yearly decrease was observed. As a whole, these results suggest the influence of different environmental factors (demographic changes, public health services distribution, and population exposure to endocrine disruptor substances) influencing the time trend of birth ratio in the Brazilian population during the last decades.

Highlights

The male proportion at birth, referred as birth ratio or sex birth ratio, is a standard measurement to assess the distribution of birth frequencies according to gender
State of Ceará State (Northeast region), with a 0.35% yearly decrease was observed. These results suggest the influence of different environmental factors influencing the time trend of birth ratio in the Brazilian population during the last decades
The factors influencing the male proportion at birth are classified as: primary, including those determining the fetal gender at the time of conception; and secondary, which in turn, influence the survival of the embryo in the maternal uterus, and are related to the neonatal vulnerability [4]

Summary

Introduction

The male proportion at birth, referred as birth ratio or sex birth ratio (male births/all births × 100), is a standard measurement to assess the distribution of birth frequencies according to gender. The factors influencing the male proportion at birth are classified as: primary, including those determining the fetal gender at the time of conception; and secondary, which in turn, influence the survival of the embryo in the maternal uterus, and are related to the neonatal vulnerability [4]. According to this classification, parental hormonal concentration right before conception is considered to be a paramount factor influencing the sex of the embryo [5,6]. The relative frequency of male births is directly related to the integrity of the parental endocrine system in a given population [5]

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