If you have any discomfort during sex, be sure to discuss this with your doctor or midwife. I have no sex drive since I've become pregnant, and my partner is frustrated. Of course your feelings regarding sex will change. Your body is being flooded with hormones. You may be growing at an alarming rate.
Many women feel unattractive, especially later in pregnancy.
Discuss your feelings with your partner. And remember that you can be intimate without making love. I just can't get comfortable during sex. If the missionary position has become more like climbing Everest, you may need to re-think exactly how you are going about things. Perhaps you may want to reverse the roles, with your partner on the bottom, instead of you.