I often hear 'look, she's pregnant'. Often they even point to the 5 year old boy when saying that.
What most visitors don't know: gorillas always have big bellies. It's normal for them. Of course not the young gorillas who still drink milk a lot. When they'll only eat vegetation like the others, their bellies will grow too. If you see a gorilla without a big belly, he or she is probably ill.
Their big belly is because they eat vegetables, grass, leaves and bark. It takes a long time for that kind of food to digest. Because of that they have a complicated (and long!!) intestinal system.
For those wondering: keepers only know gorillas are pregnant when they do a pregnancy test (the same one that's used for humans). Before doing this test, they saw gorillas mate. I think it's the same in all zoos? They write down dates when the mating was seen. And then after a few weeks they do the test.