There's medical evidence to show that fasting in pregnancy is not a good idea. If a pregnant woman feels strong and healthy enough to fast, especially during the early part of the pregnancy, she may do so. If she doesn't feel well enough to fast, Islamic law gives her clear permission not to fast, and to make up the missed fasts later. If she is unable to do this, she must perform fidyah (a method of compensation for a missed act of worship).