implantation

Meaning: Implantation is the attachment and embedding of the blastocyst in the uterine lining (endometrium), roughly a week after fertilization. Fertilization occurs in the oviduct. The zygote then undergoes cleavage while moving toward the uterus, forming a morula and then a blastocyst. The blastocyst sheds the zona pellucida, and the outer trophoblast cells secrete enzymes that digest and invade the endometrium. The trophoblast goes on to form the chorion and the fetal part of the placenta. The inner cell mass becomes the embryo proper. Trophoblast cells secrete hCG, which acts much like LH. It keeps the corpus luteum alive so it continues to secrete progesterone and estrogen, which maintain the endometrium until the placenta makes enough hormone itself.

On the MCAT: Expect developmental sequence questions (fertilization → cleavage → morula → blastocyst → implantation → gastrulation). Expect hormone graphs of hCG, progesterone and estrogen in early pregnancy, and "what if" predictions. Without hCG, the corpus luteum degenerates, progesterone falls, the endometrium sheds, and the embryo is lost. Pregnancy tests detect hCG, which appears only after implantation.

Common confusion: Implantation is not fertilization. Fertilization happens in the oviduct, and implantation happens in the uterus days later. The blastocyst implants, not the morula. hCG comes from the trophoblast, not from the pituitary, so it is not made at fertilization.