It s different depend on the woman so if you can answer on your own experience
I think the whole time span is challenging for a women. But if it is about most difficult part, then it would be the later time of pregnancy period when the tummy starts getting more heavier, more backache, more discomfort to move around and scary feeling that child shouldn't born until i lend up in hospital. And then delivery is the ultimate difficult part, the pain, the unforeseen threat of infant's health and whether its going to be normal or caesarean.