Why is this the answer?

The answer is moon because this clever riddle uses a wonderful word picture to describe how the Moon looks in the night sky. The riddle is all about the way the Moon changes its shape, which we call the Moon's phases. When the riddle says "Pregnant every time you see her," it is talking about how big and round the Moon looks during certain times of the month. The word "pregnant" means something is full and round, like a balloon that is completely puffed up. Think about the days when you look up and see a beautiful, complete circle of light. That is a Full Moon, and it looks totally whole and very full, which is the riddle's sneaky way of saying "pregnant." Even when the Moon is getting bigger right before the Full Moon (that is called "waxing") or shrinking right after it (that is called "waning"), it still looks like a very fat, round, or "gibbous" shape. It appears like a big, beautiful, rounded object in the sky, reminding us of a full belly. But here is the trick that solves the riddle: the Moon never actually gives birth to anything, because it is not a living creature. It is a giant, gray rock that orbits around the Earth. The Moon's shape only looks like it changes as it moves along its path. It will grow from a small crescent to a big, round circle, appearing very full and "pregnant." Then, it will slowly shrink back down to a tiny sliver, or even seem to disappear for a little while, only to start growing big and round again the next month. The Moon is simply reflecting the sunlight, and as its position changes, we see more or less of its sunny side. It keeps repeating this beautiful cycle of getting full and round, but it never actually creates a new baby moon. It keeps its round, full shape only for a time, then shrinks away, which is why it will never give birth.