Will the baby be male or female? If it was a random biological switch then, with a huge sample number, the sex ratio at birth would be exactly 50-50. But it isn't; the ratio is about 51-49 in favour of males. So how does this inequality arise?
Further to @Remib's comments. I was specifically thinking of the sex ratio in humans. Would it be true to say that the genetic determinant of the sex ratio would be exactly 50:50 M/F? If the disparity is environmental, then how does it work, and wouldn't an environmental effect be more variable than the 51:49% that we see?