My mother was pregnant for almost 3 months before she even knew (not with me, I was about 11 at the time). Because of various fertility problems, she was supposed to be unable to get pregnant at the time, and so my parents had basically not used any kind of protection since I was born. She missed a few periods, had some spotting, and assumed, that at about 40 or so, she was having menopause symptoms. Like **Duck Duck Goose ** mentioned, she didn’t notice until her clothes started getting too tight for no reason.
She didn’t have an abortion, but ended up miscarrying anyway (she would’ve kept it).
There are also women who have very few symptoms of pregnancy, especially for the first few months, or symptoms that are easily mistaken for other things.
Nausea can easily be explained by many other causes besides pregnancy, and many early signs of pregnancy (bloating, fatigue, etc) are also symptoms of PMS.
Also, for some women, there might be reason to keep it in the beginning - they think it will help them keep their man (bad reason, but some people still use it), they think they will have support they end up not having, etc.
And yes, some (many, I’m sure) “flip-flop” over whether or not it’s really what they want, or need, to do. While I’ve been careful and lucky enough to never have been in the situation, I’m sure it’s not an easy for most to make in a matter of weeks.