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Changes to the heart and blood vessels with pregnancy
During pregnancy, changes occur to the heart and blood vessels that add stress on a woman’s body and increase the workload of the heart. These changes include:
Increase in blood volume: During the first trimester, the volume of blood increases by 40 to 50 percent and remains high throughout pregnancy (huyết tương tăng nhiều hơn huyết cầu).
Increase in cardiac output: Cardiac output, the amount of blood pumped by the heart each minute, increases by 30 to 40 percent due to the increase in blood volume.
Increase in heart rate: It is normal for the heart rate to increase by 10 to 15 beats per minute during pregnancy.
Decrease in blood pressure: In some women, blood pressure may decrease by 10 mmHg during pregnancy. Blood pressure may decrease during pregnancy due to hormone changes and because more of your blood is directed toward the uterus



References:
http://my.clevelandclinic.org/heart/...egnancy.aspx#1
Giáo trình SẢn phụ khoa TPHCM tập 1 - năm 2007