What is implantation? When does this process begin?

Hint: Implantation is the process of attachment of the blastocyst to the uterine walls for nourishment and oxygen supply after six days of fertilization.

Complete answer:
Implantation is the stage in pregnancy at which the embryo (in the form of a blastocyst) adheres to the walls of the uterus. This adhesion to the uterine wall helps in the further growth of the blastocyst by deriving oxygen and nutrition from the mother. When the blastocyst reaches the uterus, it floats freely in the uterine cavity for two to three days receiving nourishment from the uterine secretion. Once the implantation is complete, it starts secreting hormones for preparation of the mother’s body, turning off the menstrual cycle and building the placenta.
About six days after fertilization, implantation begins. The blastocyst’s outer layer, the trophoblast, test the readiness of the endometrium of the uterus for implantation. This is determined by the level of ovarian hormones (estrogen and progesterone) in the blood. Implantation takes about a week and is completed by the fourteenth day after ovulation. The placenta formed due to implantation provides nutrition and oxygen to the embryo and carries away embryonic metabolic waste.

Additional information:
hCG (human Chorionic Gonadotropin) is a hormone secreted by the trophoblast cells of the blastocyst. hCG is detected in the mother’s blood by the third week of gestation. Hence, the common pregnancy test involves determination of hCG levels in the blood.

Note:
Implantation occurs about eight or nine days after fertilization, though it can happen as early as six days or as late as twelve days after ovulation. Successful implantation is often considered as the start of pregnancy.