One of the largest physiological changes in pregnancy is an increase in circulating blood volume. It explains a set of symptoms and test results that would otherwise appear abnormal.
The placenta requires a large flow
The placenta needs a substantial and continuous blood supply to exchange oxygen and nutrients. Meeting that demand requires more circulating volume rather than only redirected flow.
The increase begins early and builds through pregnancy, well before the fetus is large enough to account for the demand directly.
Building it in advance also provides a reserve against blood loss at delivery, which is a normal part of the process rather than an exception.
Plasma rises faster than red cells
Both the fluid component and the red cell mass increase, but not at the same rate. Plasma expands proportionally more, particularly in the middle of pregnancy.
The result is dilution. Haemoglobin concentration falls even though the total quantity of haemoglobin in the body has risen.
This is why reference ranges for blood counts in pregnancy differ from those outside it. Applying the non-pregnant range would classify a normal finding as anaemia.
The heart works harder to move it
Cardiac output rises to circulate the additional volume, through both a faster heart rate and a larger volume per beat.
Resting heart rate typically increases noticeably during pregnancy, and awareness of the heartbeat becomes more common. Both follow directly from the additional work.
This is also why exercise tolerance changes. The cardiovascular system is already operating with less reserve before any activity begins.
Vessels relax, which lowers pressure mid-pregnancy
Hormonal changes relax the walls of blood vessels, widening them. Despite the extra volume, blood pressure commonly falls during the middle months.
Dizziness on standing is a frequent consequence, as blood pools in the legs and takes longer to return. It typically eases as pressure recovers later in pregnancy.
Fluid also moves more readily into the tissues, contributing to the swelling of ankles, hands and face that many pregnancies involve.
Where the changes stop being expected
Genuine iron deficiency does occur, since demand rises alongside dilution, and screening blood tests during pregnancy are looking partly for this.
Blood pressure that rises rather than falls, sudden or marked swelling, severe headache or visual disturbance are patterns that antenatal care is specifically monitoring for.
Distinguishing expected adaptation from a developing problem depends on measurement over time by a midwife or doctor, which is what routine antenatal contacts are structured to provide.