The more children a woman has the more likely she is going to give birth to an equal number of boys and girls.

Since 1940, an average of 91,685 more male babies have been born each year than females, a total of 5,776,130 over that 63-year period.

The highest sex birth ratio occurred in 1946 (1,059 male births per 1,000 females) while the lowest occurred in 1991 and again in 2001 (1,046 male births per 1,000 females).

There were three major trends in sex birth ratios over this period: a significant decline between 1942 and 1959, a significant increase between 1959 and 1971 and another significant decline between 1971 and 2002.

Combining all the years studied, older mothers (40 to 44 years of age and 45 years and up) have the lowest total sex birth ratios (1,038 and 1,039 respectively) and mothers 15 to 19 years of age had the highest sex birth ratio (1,054).

For all available years combined, Chinese (1,074) and Filipino mothers (1,072) had the highest differences between the number of boys born versus girls, while non-Hispanic black (1,031) and American Indian mothers (1,031) had the lowest.