What is implantation?
Implantation is the process in which the blastocyst (the fertilized egg) attaches itself to the uterine wall and begins producing hCG. Production of hCG enables the blastocyst to attach itself to the uterine lining. hCG is the hormone that is typically measured in pregnancy tests.
An egg is usually fertilized in the outer part of the fallopian tube within 12 hours of ovulation. After fertilization the egg travels down the fallopian tube, growing as it goes. The egg doubles, then grows to four cells, then to eight and as it enters the uterus it becomes the blastocyst. This process happens about eight days after ovulation.
Implantation typically happens in the upper third of the uterus and can sometimes cause bleeding, referred to as implantation bleeding or implantation spotting. This happens when the blastocyst burrows into the lining of the uterus and is considered normal. Implantation bleeding can occur around the time you might expect your period. After implantation the blastocyst will continue to grow and will soon become an embryo.