I sometimes say "If I get 50 eurocent per person I hear say 'look that gorilla is pregnant' I'd be very wealthy after spending a day in the zoo" Of course that's when it's a weekend or Holiday day. When people say that, they often also point at young males (between 4 and 7 years old)
What most people don't know: gorillas always have big bellies. It's typical for them. Okay, young gorillas who still drink a lot of milk don't have these big bellies yet. When they slowly start eating more and more food, their belly starts growing too.
If you see an adult gorilla without big belly, he/she is most likely ill. There are exceptions of course. Both in GaiaZOO and in Zoo Antwerp there are slender gorillas. In GaiaZOO that's female Sangha and teenager Loango. With Loango it's a 'medical' cause: he's castrated on a young age. In Zoo Antwerp female Mayana is very slender.
They have big bellies because they eat plant-based food. That food neds more time to digest. That's why they have a complicated (and long!) digestive system. They have about 5 meter (!!) more intestines. And well, some gorilla have bigger belies than others; mostly it's because they love to eat and eat a lot
How is it possible for gorilla keepers to see a gorilla is pregnant? They can't see that. The only way to know a gorilla is pregnant, is by doing a pregnancy test (the same used for human women). Most of the time they saw gorillas mate. They then write it down and a x number of weeks later they do a pregnancy test. It did happen in the past that there were surprise babies. I don't remember where that happened, but it was in 2 zoos in Germany if I'm correct.