During pregnancy, a woman's average heart rate may raise from 70 beats per minute to 80 or 90 beats per minute, according to Merck Manuals. This increase in heart rate allows the heart to pump extra blood to the uterus and the fetus.
During pregnancy, a woman experiences a 50 percent increase in her total blood volume. All of this extra blood must be pumped through her heart in order to reach the baby's circulatory system and reach the uterus. The extra blood causes the woman's heart to increase its cardiac output. In turn, the woman's heart rate increases, states Merck Manuals.