Your Baby's Development at Week 3
During the latter part of the third week of pregnancy, the fertilized egg divides into hundreds of cells and is now called a blastocyst. The blastocyst releases the hormones that will stop your menstrual cycle. When it reaches your uterus, the blastocyst will implant itself into the inner lining of the uterine wall, which is also called the endometrium.
The endometrium will provide the developing embryo with nutrients, and it will remove waste. Over time, this implantation site will develop into the placenta, which will nourish and protect your developing baby throughout the pregnancy. By about 10 days after fertilization, the blastocyst is firmly implanted into the endometrium.