The fertilised egg (the blastocyst, zygote) enters the uterine cavity from the fallopian tube on the 4th day of fertilisation and by day 7, it starts embedding itself into the prepared endometrium (now called the decidua), the process known as implantation.

If the embryo survives (implants), then pregnancy begins. However, the embryo may not implant, which means it survives only for a few days. In this case, it sheds from the uterus as the endometrium breaks and passes out via the vagina in the menstrual fluid.

This early loss is very common, and the woman does not even notice that she was pregnant. Implantation may also occasionally occur in the fallopian tube, which is called an ectopic pregnancy (pregnancy outside the uterus) and is very dangerous for the woman. The tube may rupture as the embryo grows, and cause severe pain and loss of blood into the abdominal cavity.

If the blastocyst implants successfully in the uterus, the cells go on multiplying by cell division and moving around into new locations to form two distinct structures.

• Three or four blastocyst cells develop into the inner cell mass, which, over the next few weeks will form into the recognisable structures of a human embryo, with a head, beating heart and tiny limbs. Some of these cells also develop into the fetal membranes that form a fluid-filled protective 'bag' around the embryo.
• The remaining 100 or so blastocyst cells form a structure called the trophoblast, which will provide the baby's contribution to the placenta.

The first stage of development of the placenta is when the trophoblast cells burrow into the endometrium (see figure 3.2. below).