Over the course of the pregnancy, dramatic changes occur in the mother and fetus.

There is an increase in the total maternal blood volume of about 40%.

Matching this increase in maternal blood volume is an increased cardiac output of about 40%, maxing out at the end of the second trimester. For those women with a pre-existing cardiac impairment, the middle trimester is the time of greatest danger because that is when cardiac overload, if it is to occur, will most likely occur.

The resting pulse is usually slightly increased during pregnancy, and the pulse pressure is normally a little wider during pregnancy, due to a decreased peripheral resistance.

Blood flow through the kidneys increases, with an accompanying increase in glomerular filtration rate.

Because the GI tract extracts calories more efficiently from dietary sources during pregnancy, weight gain normally occurs. The Institute of Medicine recommends a target of 25 to 35 pound weight gain for women of normal weight prior to the pregnancy. They recommend somewhat more weight gain for underweight women, and somewhat less weight gain for overweight women.