In the first trimester, not surprisingly, all of those early pregnancy symptoms such as nausea can definitely dampen desire. That is why so many women find they have a limited libido in the first trimester. The second trimester is in general more comfortable for all kinds of activities, so during that time many couples seem to enjoy sex relatively more than they do during the other two trimesters. In the third trimester you have the sheer gymnastics of trying to get around that mountain of a belly, and that takes quite a bit more effort. So again, there is increasing fatigue in the last trimester.
Some women enjoy sex during the entire pregnancy, though they are a minority.
Some women find that the increased blood flow to the genitals makes it a lot easier to have an orgasm. There is also the matter of the extra lubrication which nature provides you with when you are pregnant. That can be a good thing if you are ordinarily too dry, or that can be a bad thing if sex becomes just too slippery. Since men don't experience all of the same physical changes that women do, they may not experience any change in their libido, although some men certainly do. What's probably called for in your relationship is some open communication about how you feel, because no one should be forced to have sex if they don't feel like it.