So, Kay doesn't know if her child is male or female, right? But because she wants to end the Sicilian thing, she takes a chance that it's a son?
Why do you assume she didn't know? In the film she makes very clear that she did know, and it's very clear that Michael believed her.
Just because Tom makes an offhand comment about "three and a half months" doesn't seem to me to be a strong argument in favor of a miscarriage.
Maybe Tom was wrong about when the baby was conceived. Maybe one of Michael's forays into the legitimate world was in cutting-edge obstretrical technology. Maybe FFC didn't really think obsessed fans like us would parse the films to this extent to come up with theories.
All of those make more sense than Kay making up a medically impossible story and Michael buying it. (OK, my second theory makes less sense.)
