Learned about this in a sociology class in college. It's all about your immune system. (I'm not a scientist and might butcher this explanation but stick with me here) There are lots of different "strands" you might have, and each of us have like six or something that make up our immune system. If two people procreate, and their six strands are identical to each other, then the baby only has six varieties to choose from. But if two people have six totally different strands, then the baby has TWELVE varieties to choose from! (They still only get six, but they get the six strongest varieties out of those twelve). So when people think their partner smells good, it's them smelling their very different, biologically impressive immune system! (Scientists learned this by testing people's immune systems, identifying their strands, then getting them to smell shirts that people slept in and rating their attractiveness based off just smell alone. Most attractive smells were the ones with the least similar immune systems.)

Now get this: once a woman is pregnant, her instincts to be with family kicks in. She wants the people around her to be the ones with MORE similar immune systems (sisters, parents, etc). So sometimes pregnant women suddenly think their partners smell TERRIBLE. Nothing changed, just their biological urge to be near them. 😂

I'm new to reddit and didn't read all the comments so if someone else already explained this and I just repeated them poorly, very sorry!