Influence of parental and biological factors on the male birth fraction inthe United States: an analysis of birth certificate data from 1964 through1988

Objective: To determine the role of parental and biological factors on the U.S. male birth fraction from 1964 through 1988. Design: Logistic regression on annual U.S. male births by race group. Setting: Population-based data. Patient(s): Live births in the United States 1964 through 1988. Intervention(s): None. Main Outcome Measure(s): Annual U.S. male birth fraction by parental and bi ological factors. Result(s): During the study period, the annual U.S. male birth fraction sho wed changes based on race group, parental age, and low birth weight. The ov erall influence of parental age on the U.S. male birth fraction is strong a nd is stronger in nonwhites than in whites. The U.S. male birth fraction is also strongly influenced by the percentage of low birth weight infants in nonwhites, but not in whites. The male birth fraction declines with increas ing age of either parent and with an increase in the percentage of low birt h weight infants. Conclusion(s): The relative magnitude of influences on the U.S. male birth fraction depend on the race group, which may be a reflection of the range o f observed data rather than biological differences. The developed models ha ve reasonable predictive power and are an appropriate first step in underst anding the factors influencing the male birth fi-action. These types of par ental and biological variables should be included in models before examinin g other exogenous and population level variables. (C) 2000 by American Soci ety for Reproductive Medicine.