Tomorrow is Valentine's Day and, as an obstetrician, I believe that one of the biggest sacrifices that a woman makes for love is having a baby.

While many women independently want children, a large number of women come into my office desiring a child because of their partners' wishes.

Remember Paul Anka's very popular song: "Having my baby, what a lovely way of saying how much you love me".

Before the challenges of childbirth, pregnancy results in changes to a woman's body. Her womb can grow to about 15 times its normal weight and can expand to about 1,000 times its normal volume.

The structure of the womb also undergoes considerable changes. It changes from a compact thick-walled organ to a large muscular organ with much thinner walls.

The neck of the womb or cervix softens and both the vaginal walls and the cervix might appear bluish in colour.

Changes in the mother

A pregnant woman's breasts may become tender and she may experience tingling. Both her breasts and her nipples increase in size and during the pregnancy she may notice early signs of milk production. Retaining water is a normal change in pregnancy and she will retain at least six litres of extra water.

This is part of the reason why some women have such significant swelling in their feet and sometimes hands. Her blood sugar levels are very different after pregnancy and they change from being higher than normal after meals to significantly lower than normal levels when she is hungry. A pregnant woman may experience nausea and dizziness if she has not eaten for a long time.

Other changes include an increase in the volume of blood in a pregnant woman's body which in some women is almost twice the normal amount. The volume of blood which the heart pumps around the body also increases the function of the lungs is normal in a healthy pregnant woman, she may experience the sensation of shortness of breath.

Heartburn which pregnant women commonly experience is also due to pregnancy-associated changes. These include the shift in the position of the stomach because of the growing womb and changes in pressure of the stomach and the oesophagus.

Changes in the 'foetus'

But who or what is her body changing for? Early in her pregnancy before she feels any movement or notices any large changes in the size of her womb she might wonder about the reality of her pregnancy. The developing baby has a very important role from the beginning in determining its own future.

It is the fertilised egg, now called the blastocyst, that is the main force that determines its implantation in the womb. It is this early 'foetus' that stimulates the signals that allow a woman to know that she is pregnant. The foetus has the ability to harness the resources that its mother provides to ensure its survival. It is itself undergoing rapid changes.

By about six weeks of pregnancy there are already arm and leg buds and the heart begins to divide. Two weeks later, the heart is fully formed and the embryo (the name given to the developing baby between four to 10 weeks of pregnancy) has fingers and toes. At 10 weeks, the developing baby is referred to as the foetus. By 12 weeks the foetus is already moving and in just another two weeks, the sex can be determined by an ultrasound. Therefore, even before the pregnant woman can start to feel movements, it is already a boy or a girl.

This sacrifice of love is one that can result in an unparalleled reward.