A pregnancy usually lasts forty weeks, or close to nine months. This time has been divided up into three ‘trimesters’. Each trimester lasts about three months (or thirteen weeks). During the first trimester (zero to thirteen weeks) conception, implantation in the uterus, and formation of a placenta and embryo occur. In the second trimester, more organ development occurs, and in the third trimester more nutrients are taken on board and final maturation of organs takes place.
Various words are used to describe the inhabitant of the uterus. Very early on it may be referred to as a fertilised egg, or conceptus. It is called an embryo during the first eight weeks of development, and after eight weeks it is called a foetus, until it is born and we know it as a baby.
When the egg is fertilised in the fallopian tube, and settles in the uterus six to eight days later, the ovary which has produced that egg has the responsibility of supporting the conceptus, hormonally, until it can make its own. The major hormones needed are oestrogen and progesterone, so the ovary churns them out of its ‘corpus luteum’, which is the sort of cyst which is left after the egg has popped out of the ovary.
So there is extra oestrogen and progesterone around, causing several changes in different parts of the woman’s body.
Meanwhile, back at the uterus, the conceptus is busy multiplying its cells, becoming an embryo, a sac and a placenta. The placenta is an incredibly clever and essential piece of equipment. It is made up from cells of the conceptus, as well as from the uterus lining (endometrium) to which it is attached. The placenta’s function is to transfer oxygen and nutrients to the embryo, and remove wastes. It does this by allowing the blood vessels of the embryo (via the umbilical cord) to come into contact with those of the woman, although the actual blood itself does not mix. As well as acting as an exchange plate for the woman and her embryo, the placenta also starts making oestrogen and progesterone to support the pregnancy.
The embryo grows at a predictable rate in early pregnancy. At six weeks from the last period it is about 0.4 centimetres long. By eight weeks it is about 2 centimetres long, and weighs 1 gram. At twelve weeks the foetus is over 6 centimetres long, and weighs about 15 grams. During this time development of various parts and systems is progressing. It is becoming more recognisable as human, with arms and legs, and a relatively large head.