The increase in blood volume that takes place during a woman’s pregnancy happens for two reasons.
Firstly it facilitates both fetal and maternal exchanges of nutrients, respiratory gases and various metabolites that take place.
The second factor is so that when a woman naturally loses a lot of blood during the process of delivery the impact on the body will be reduced.
Blood loss can be up to 500 milliliters during a normal birth and up to one thousand for a caesarean section.
This is compensated by what is known medically as the ‘autotranfusion’ of blood from the contracting womb.
Cardiac output is similar to blood volume in that it increases by a similar amount during gestation. In the first cycle it is over a third higher than normal and rises steadily.
From a starting point of 6.7 liters per minute at the end of the last trimester it will be much higher at roughly 8.7 liters per minute. The reasons for this are a more rapid heart rate and more importantly the substantial increase in stroke volume.
The actual heart itself enlarges during pregnancy. As the uterus expands the diaphragm is displaced causing the heart to be pushed sideways and forward meaning that the apex is changed to outward and upward positions.
From the middle of gestation the engorged womb will compress the inferior vena cava and also the lower aorta when a woman is lying down.
The final trimester sees a marked lowering of kidney function but this is more evident when the expectant woman is lying rather than if standing in an upright position.