Was Sekai in School Days anime really pregnant? In the final episode of School Days anime, Sekai said that she was pregnant. Her behavior seemed like she was telling the truth. 

However, when Kotonoha offered Sekai to consult at the hospital, she refused to go. This made Kotonoha suspect that Sekai was lying. And when Kotonoha stabbed and dissected Sekai, Kotonoha confirmed her suspicion.

But even before her death, Sekai stated clearly that she was pregnant, though she didn't have to lie to Kotonoha anymore as Makoto was already dead.

So was Sekai really pregnant or not?
It's not definite, and you'll hear differing opinions based on who's talking about it:

She was pregnant:


She had morning sickness
Kotonaha's attempt at searching inside Sekai is of course fruitless, as the baby would not have had enough time to develop. Maybe after several weeks something would be visible. Also it's a very crude way of doing it, so it's not surprising she wouldnt see anything, not being a trained professional 
She didn't want to go to the hospital understandably, it would be a frightening and stressful experience.
She had no motive, she was clearly worried by the experience.


She was not pregnant:


Morning sickness started occuring too early
There have been several instances of fake pregnancies, where the body emulates all the symptoms of pregnancy (sickness, missed periods, etc) because of stress / belief that they are pregnant
She was scared that her lie would be revealed if she went to the doctor
Her motive could have been that a baby would tie Makoto strongly to her.


Sekai Wikia page

I doubt you'll find a conclusive answer, as many posts out there have already made up their mind. But one thing I think we can gather is at least she thought she was pregnant, regardless of if she was or not.