Implantation of the embryo is the attachment of the embryo to the endometrium (lining of the womb). It occurs about a week after fertilization; and is a very delicate and complex process.
Once the embryo reaches the uterus, it may float around for 2-3 days, still growing. It does not stick to the lining of the uterus until about 7 days after fertilization when the process of implantation starts. By this time the embryo, now known as the blastocyst, will have two distinct types of cells: one type is called the inner cell mass comprising stem cells that will form the fetus, and the other extra-embryonic cells called trophoblast which form the placenta (afterbirth) and control stem cell development as the embryo develoips. The embryo begins burrowing into the endometrium and the placenta starts to form - a pregnancy is achieved. At this stage, the placenta starts to produce the pregnancy hormone (human chorionic gonadotropin - hCG).
This hCG will continue to stimulate the corpus luteum so that progesterone production is maintained and the pregnancy can continue. The corpus luteum is functional for the first 8 weeks of pregnancy then the progesterone production is taken over by the placenta.
The most common place for the blastocyst to implant is at the top and back of the womb.
Not all embryos implant and not all implanted embryos will further develop to fetuses, and not all fetuses end in healthy babies. Sometimes adverse outcomes can result, such as miscarriages and ectopic pregnancy. However, the majority of couples will end in having healthy babies.