4 weeks pregnant, or 2 week after conception: implantation of the egg

The fertilised egg, measuring about 1.5 mm, implants in the uterine lining. This stage is called implantation and lasts a few days. Some bleeding may occur during this stage, but it is not serious.

A hormone is then produced by the egg: the chorionic gonadotropic hormone (HCG), which can be detected by pregnancy tests and is essential to maintain the pregnancy. At this stage, the egg is made up of two layers: an outer part that will become the placenta and an inner group that will become the embryo.