Welcome to Gaia! ::

Some women take their birth control pills without hormone breaks for long periods of time.. so they skip over have a menstral period. You could look into that possibility.

Wow. How stupid can people get? *faith in human race goes down a notch*

Oh forgot to add-

Teenage mothers are another source of "not knowing being pregnant untill it pops out"
Remember that case not too long ago that they had all over the news about how a neighbor found a newborn in her backyard and they were trying to hunt down the mother whom they thought to be a scared teen and then all the newstations like CNN/MSNBC were having people come on and talk about (s**t forgot what it's called) but where you can leave a newborn at a hospital/fire&rescue/police etc. and not be jailed.

Well anyways.
Teens having sex, especialy those with strict, non-understading parents may be in denail if they become pregnant.
sure they might have their period stop and the whole works, but because of the home environment, they refuse to accept that they are pregnant and then up giving birth secretly in their beds and dumping the newborn babes out of fear.

It's not about being stupid. Maybe being silly and not checking up on things like you should be, but there are many women that have really wonky cycles and don't notice when it's be a few months since their last menstrual period. Then you have pregnant women that don't exhibit any real signs of pregnancy such as morning sickness, weight gain, etc. And even those that do have some kind of symptoms such as the nausea, a lot of those things can be explained away as just common illnesses such as the flu.

Even in a healthy pregnancy, not all women will develop the obvious "pregnant belly" that everyone associates with gestation. I wish I had a link to it, but I found an article awhile ago that had a picture of three different women with different sized bellies. One looked like she was maybe at the end of her first trimester, the next looked like she was around six months or so and the third looked like she was ready to pop. The article asked to identify which woman was at which stage of pregnancy. Turns out all three of them were at about five to six months along.