Some women maintain their drive for sex throughout their entire pregnancy, and some do not. Every pregnancy is unique, and what is best for one may not be for the other. In the beginning of your pregnancy you may experience morning sickness or an overall feeling of fatigue. This may lead to much less, if any, desire and that is fine.
As your levels of hormones (estrogen and progesterone) increase, your body may have a boost in libido. Estrogen especially is a factor in this because it is increasing the blood flow down to your uterus. In turn, your vaginal may become more lubricated and your breasts will become more sensitive. Many women take advantage of these sensations and have sex regularly during pregnancy.
Other factors during pregnancy, as it progresses, may inhibit your desire for sex. Hormonal changes, weight gain, back pain, and other common pregnancy symptoms and ailments may keep you out of the mood. Emotions can play a large factor, too, as self-confidence issues as your belly increases in size can affect your wanting for sex. A fear of sex harming the baby may enter your mind; however, having sex in a normal pregnancy will not harm the unborn child. Your cervix and the amniotic fluid give the baby full protection.
When talking to your physician about sexual activity, your doctor will make clear of any risks you may have, such as a history of preterm birth or labor, vaginal bleeding that is unexplained, or if you have cervical incompetence. Also, if you are leaking amniotic fluid or if you have placenta previa then you will be told to abstain from sexual activity.
If you are cleared for sex, then go for it (if you are up to it!). Of course, as you become more advanced in your pregnancy, you are going to find it necessary to try different positions. Positions with you on top, on your side, or other creative ways can keep sex fun and enjoyable for both of you. What is important is that you continue your loving relationship during and after your pregnancy, whether you participate in sex or not. Either way, enjoy each other and savor the last few months of just being the two of you!