Grant et al (2008) suggest that there may be a maternal influence on the sex of the infant. Eggs developed in a higher-testosterone follicular environment were more likely to be fertilized by Y-chromosome bearing (male) sperm, suggesting that the eggs were preferentially inseminated by these sperm.
Whether this is possible has been argued for some time. Trivers and Willard (1973) argued that more fit mothers would produce more sons, because a more fit male specimen is likely to out-reproduce a more fit female individual, and the reverse is true when they are less fit (parental investment). Clutton-Brock et al (1984) found that higher-ranked female red deer bore more sons than their subordinates. However, Silk et al (2005) found that maternal rank did not influence sex ratios in baboons.
Trivers and Willard pointed out that as women got older, they were more likely to bear daughters -- but later critics said that their argument didn't hold up. I didn't examine it closely enough to weigh in either way. For what it's worth: I'm first-born.
I'm more than a little irked over the testosterone-equals-dominance thing, but since that's sort of like being irked that the sky is blue, I'm trying to ignore it.