I am not sure if all pregnant women prefer to eat bitter foods. Personally I think pregnancy is unique for every woman. And I have heard that even between pregnancies a woman will have unique experiences, so the first pregnancy is different from all the subsequent pregnancies. In my personal experience, I never had any cravings for particular foods. You hear a lot of stories about how women will eat soil, or even bitter or sweet things. I think pregnancy is a unique experience and your body as woman experiences a lot of hormonal changes suddenly and over the entire course of the pregnancy and even after giving birth. So it is not that women prefer to eat bitter foods but they try to accommodate their current changing body environment.
What the examples of the foods you have heard pregnant women eating?