During your pregnancy your desire for sex fluctuates and every woman is different. My best friend didn't have sex the entire time she was pregnant because she was so sick all the time and she started experiencing preterm labor at 6 months. Luckily she made it full term she was just miserable throughout her pregnancy.
Most women experience morning sickness and fatigue in the first trimester that sex happens less frequently than before pregnancy, but by the second trimester you start feeling better and have more energy so things start to return to normal.
Some people wonder if sex is safe through pregnancy and most doctors will say yes unless there are other problems or complications with the pregnancy. However, keep in mind as baby gets bigger and belly gets bigger discomfort becomes an issue. When baby begins to drop it becomes an even bigger issue. This is a good time to try different positions that may make it easier. It is important for pregnant women to communicate discomfort to their partner.
Communication is important for couples anyway, but when it comes to sexual intimacy it is even more important. Lack of desire is common in pregnant women and should be communicated. Sex is not an obligation to your partner it is a way to connect with your partner so if you're not feeling up to it don't do it, but communicate that to your partner in a way that doesn't make him feel rejected. Any good husband or boyfriend understands that your body is working very hard building a baby.