At the beginning of the third week of pregnancy there is ovulation and fertilization. The woman has not missed her period so she will not know that she is pregnant.
On the first day, at 16-20h post-fertilization, the zygote appears. On day two, the embryo has four cells, day three eight and day fourteen, at this moment it is called a morula. The fifth day follows the increase in cells and is called a blastocyst.
On the sixth-seventh day the embryo reaches the uterus and begins its adhesion to the endometrium, implantation. Normally the woman during this week does not have any symptoms.