This breaks my heart, I'm so sorry. I'm partial to the name Stephanie, too.
My SIL didn't find out she was pregnant with my nephew until she was about 6 months along. She was married to my brother and they already had an older child. She didn't have regular periods, and didn't have any other symptoms until she went to the ER with really bad abdominal pain (turned out to be gallstones).
I am friends with a couple who are ministers (captains) in the Salvation Army. As part of their employment they have to have periodic physicals. She went for her physical, and they noticed something "unusual" with her bloodwork. They sent her for an ultra-sound, and she was 30 weeks pregnant. She found out she was pregnant and the sex of the baby all in the same day. If it hadn't been time for her physical, who knows when she would have realized? He is their 6th child, so she was hardly a first-time mom.
I think there are people who don't know because they don't want to know, and would rather just ignore it. But I think not knowing for a long time is more common than people think.