The sex ratio in the USA has declined over recent decades, resulting in fewer male births. Concurrent changes in the childbearing population may have influenced the sex ratio, including increases in multiple births, improvements in perinatal survival and increased Hispanic births.Data from the US natality files (1981-2006) were analyzed to determine the impact of changes in birth characteristics on male birth proportion. Male birth proportion was calculated as the number of male births divided by the total number. In separate analyses, trends in male birth proportion from 1981 to 2006 were adjusted for plurality (singleton, multiple), gestational age (or=37 weeks) and, from 1989, maternal Hispanic ethnicity. Separate analyses were conducted for white and black births. Log binomial regression was performed to estimate crude and adjusted trends with year as independent variable.Trends in male birth proportion differed significantly according to plurality among white (P < 0.01), but not black births. Adjustment for gestational age tempered the trends among white singletons (P < 0.0001) and multiples (P < 0.05) but had no effect on trends in black male birth proportion. Adjustment for Hispanic ethnicity had no impact on trends in black male birth proportion and any effect on white births was negated by changes in gestational age trends.Lack of consistent influences on, or patterns of change in, the proportion of male births between different subpopulations of births suggests that a single mechanism is unlikely to explain the oft-referenced decrease in the overall US sex ratio.