7.4.2  Blood volume

Blood volume (the total volume of blood in the circulation, measured in litres) increases gradually by 30-50 % in the pregnant woman, so by full term she has about 1.5 litres more blood than before the pregnancy. A higher circulating blood volume is required to provide extra blood flow through the placenta, so nutrients and oxygen can be delivered to the fetus. The increase in blood volume is caused by two changes:

  • Increase in the volume of blood plasma (the fluid part of the blood).
  • Increase in the number of red blood cells in the circulation.

The volume of blood plasma increases after about the sixth week of pregnancy. It reaches its maximum level of approximately 50% above non-pregnant values by the second trimester, and maintains this until full term.

The total volume of red cells in the circulation increases by about 18% during pregnancy, in response to the extra oxygen requirements made by the maternal, placental and fetal tissues. Red blood cells contain the oxygen-carrying substance called haemoglobin, which is rich in iron (see Box 7.1). Taking iron supplements during pregnancy can result in a much greater increase in red blood cells, up to 30% more than non-pregnant levels.

Haemoglobin is pronounced ‘heem oh gloh binn’.

Box 7.1  Iron, haemoglobin and anaemia

Iron is present in all cells and has several important functions, including oxygen transport and storage in the human body. It is the critical component of the oxygen-carrying substance haemoglobin, found in all red blood cells. It is the iron in haemoglobin that makes these cells appear red. If the diet is too low in iron, the person cannot make enough red blood cells. Iron is also involved in the storage and release of oxygen in the muscles.

Anaemia is most accurately defined as a low concentration of haemoglobin in the blood, but it is often referred to as a low concentration of red blood cells. Too little iron in the diet is the leading cause of anaemia.

The measurement of haemoglobin is expressed using its chemical symbol (Hb), and its weight in grams (gm) per decilitre (dl) of blood. A decilitre equals 10 millilitres (ml). The World Health Organization recommends that the pregnant woman’s haemoglobin should not fall below 11 grams of haemoglobin per decilitre of blood (Hb 11g/dl). (You will learn more about anaemia and its treatment in Study Session 18.)

Although there is a constant increase in the number of red blood cells in the circulation during pregnancy, the increase in the volume of blood plasma is much larger. So even though the pregnant woman has more red blood cells than before she was pregnant, they are diluted in the much larger volume of blood plasma.

  • What effect will this have on the concentration of red blood cells and haemoglobin in the pregnant woman’s blood, compared to her non-pregnant self?

  • The concentration of red blood cells and haemoglobin will fall because they are more dilute, so the woman’s blood will be slightly anaemic.

This effect is referred to as physiological anaemia. It explains why iron in the diet, or from iron tablets, is so important during pregnancy.

7.4.3  Blood pressure in pregnancy