How can you be 7 months pregnant and not know?

We have a rule in my choir class that what is said in class, stays in class. Besides the father of the baby, we were the first people she told. You could tell she really needed to talk about it. She's actually one of the most mature students in my class so it is really shocking she didn't know. The father is already 18 and has an apartment, he wants to keep the baby, but she's going back and forth from keeping it to adoption.

If she was taking her BC pills and having what she thought were periods, there was no reason for her to even think she was pregnant. Maybe the baby isn't/ wasn't very active, she wasn't taking prenatals. I don't think it would be too hard not to know in her case, especially if this is her first. Even if she felt little flutters or something she might not have known what they were. I knew I was pregnant and kept feeling "something" but not what I thought kicks should feel like so I didn't think the baby was moving much. But then someone on here was explaining what it felt like when her baby moved and I had an Ah-ha moment. So I don't think it's that farfetched that she didn't know. I'm not saying she's completely innocent, just that it's not a far stretch for her to have been in the dark about it. Now the cases that I have a problem with are the ones where the lady already has a couple kids, how could she not know?