Caution: work in progress!

You may still have no symptoms that make you feel pregnant, and life may seem unchanged. Inside you, however, a factory is working at an unimaginable speed: every embryonic cell is continually active. Symptoms are gradually beginning to appear. Like many women, you may feel nauseated on waking and may even vomit. Food smells that once appealed to you may now be unpleasant, or your partner's favourite perfume may suddenly seem disagreeable. A colleague's cigarette may be more irritating than ever. The smells have not changed; you have. Pregnancy hormones stimulate the brain's nausea centre, producing emesis gravidarum. If nausea and vomiting are severe enough to interfere with daily life and nutrition, this is hyperemesis gravidarum, and medication may be needed.

This is a critical period for organ development, so avoid potentially harmful substances such as alcohol and tobacco. Many women share the news with friends and family this week. Others prefer to wait until the period of miscarriage risk has passed and normal development has been confirmed. Both approaches are normal.

At 21-23 days after fertilisation, the embryo is about 1.5-3 millimetres long. Growth and change are rapid. There are 4-12 pairs of somites, and the beginnings of the eyes and ears have formed. The neural tube starts closing. The neural crest begins forming the skull and head. The two endocardial tubes join into a single S-shaped tube. This asymmetrical primitive heart begins contracting: the embryo's heart has started beating. However, the vessels are not fully developed, so circulation is not yet complete.

The embryo is attached to the developing placenta by the connecting stalk. The central nervous system is the most developed system, and the forebrain has completely closed. By the end of day 27, the embryo is C-shaped, with distinct folds that will form the face and neck. Septa dividing the heart's chambers appear and valves begin developing. Cells of the digestive system differentiate to form the liver, stomach and pancreas; the liver cells are the first to do so. The embryo is now about 5 millimetres long.