Scientific American:

The study looked at women in Finland. The women’s microbial makeup changed dramatically between the first and third trimesters. The array of microbes in the gut went from looking normal in the first three months of a pregnancy to resembling what’s found in patients suffering from metabolic disease in the last three.
The reason for this change is not understood, but scientists suspect it has something to do with the immune system. And now that you are reading this closing sentence, know that I'm ending this post without saying a mean thing about pregnant women.