I think there's some good animal examples of stuff like this I just can't remember what animal(s) do it.

I think it's already known that female embryos are stronger as are female children, less likely to die, more viable, etc. I've heard about 50% (I might have that number wrong) of embryos are spontaneously aborted at such an early stage that no one was even aware they were pregnant. So even a slight skewing of that number towards one gender would be significant.

I think you hit the nail on the head, either one or both of those pathways would be how the gender could be favored.

Ah! My brain is better than I thought: http://www.nlm.nih.gov/medlineplus/...icle/001488.htm
"It is estimated that up to 50% of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among known pregnancies, the rate of miscarriage is approximately 10% and usually occurs between the 7th and 12th weeks of pregnancy."