This thread is very interesting.
I'm the youngest out of 5 children, I have three sisters and one brother, though my brother is deceased (not due to any illness or pregnancy-related thing).
I was the biggest out of all of my sisters, I weighed 7 pounds and 4 ounces, whereas my sisters all weighed under 7 pounds (5 pounds 10 ounces, 6 pounds and 6 pounds and 2 ounces). My brother weighed in at 7 pounds and 10 ounces. I think I was born about a day before the due date, and from what I know my mum was under a fair amount of stress a lot of the time. She said I wasn't born with any sex ambiguity, and she wasn't taking any medication to prevent miscarriages or the like.
She did however have cervical cancer about a year before she was pregnant with me (she got part of her cervix removed before it spread), and she thought she wouldn't be able to have any more children after that. She has said that she always thought she was going to have a baby boy and she was rather surprised when they said it was a girl.
I'm wondering if cancer could have anything to do with it? As far as I know that was the only reproductive-related thing she had to get treated around the time I was conceived and born. Even if it doesn't, it's still quite interesting reading all of this thread.