Important Facts for Sex Ratio at Birth

Definition

The sex ratio at birth is the comparison of male births to female births (term singletons only). The expected sex ratio at birth (male to female) is 1.05, or 5% more males born than females.

Numerator

Number of live male births in a specific period of time

Denominator

Number of live female births in a specific period of time

Why Is This Important?

The expected sex ratio at birth (male to female) is 1.05, or 5% more males born than females. Population growth is, in part, related to the number of live male children. Numerous studies have reported changes in the ratio of males to females at birth; many of the studies have found a reduction in male relative to female births in different countries throughout the world.

Other Objectives

Some factors can affect the sex of a newborn. Decreases in male births have been shown to be inversely related to parental smoking, gestation length, parental age, and birth order. Reproductive practices and social constructs regarding sex preferences - males over females, for example - can affect the observed sex ratio. Although the mechanism which determines the sex of the infant is not completely understood, some environmental studies have suggested that environmental hazards can affect how many males are born. Biological parent(s) and/or the fetus can come in contact with and become exposed to different hazards referred to as endocrine disruptors. Fewer males are conceived when exposure to endocrine disruptors results in a decrease in testosterone.

What Is Being Done?

The Office of Vital Records and Statistics at the Utah Department of Health records all birth records along with their associated sex.