7.4.1  The heart

The heart may increase in size during pregnancy due to an increase in its workload.

  • Why do you think the workload of the heart has to increase?

  • Because it has to pump blood through the placenta, fetus and the much larger uterus and abdomen of the pregnant woman.

The amount of blood that is pumped out of the heart each minute is called the cardiac output. Table 7.1 shows how it increases during pregnancy.

Table 7.1  Changes in cardiac output during pregnancy

Woman’s conditionCardiac output (litres per minute)
non-pregnant, resting2.5
end of 1st trimester5
end of 2nd trimester6

The increase in cardiac output is caused by two changes in how the heart functions:

  • Increase in the resting heart rate, i.e. the number of heart beats per minute. The heart rate is about 15 beats per minute higher in the pregnant woman.
  • Increase in the stroke volume, i.e. the volume of blood pumped out of the heart in a single heart beat. It is about 7 millilitres (ml) larger per heart beat in the pregnant woman.

Cardiac output is calculated by multiplying heart rate and stroke volume.

During the second trimester of pregnancy, the mother’s heart at rest is working 40% harder than in her non-pregnant state. Most of this increase results from a more efficiently performing heart, which ejects more blood at each beat.

7.4  Changes in the cardiovascular system

7.4.2  Blood volume