The morula reaches the uterus between the 3rd and 4th day. Around the 5th day the blastocyst emerges from the zona pellucida and comes directly into contact with the endometrial epithelium (stage 4). The trophoblast cells begin then to multiply and form a mass of cells, the syncytiotrophoblast (syncytium = no boundaries between cells) whose processes penetrate through the uterine epithelium and invade the connective tissue of the endometrium. The blastocyst embeds itself in the uterine wall on approximately the 7th or 8th day. This is an interstitial implantation since the embryonic complex penetrates completely into the maternal connective tissue that is highly perfused with blood (stage 5a).

On the 9th day the uterine epithelium completely covers the implantation location again. The neighboring cells of the uterine endometrial stroma which are under the influence of progesterone released by the corpus luteum react to the presence of the blastocyst. They become metabolically and secretionally active. One now designates them as decidual cells.

The development of the placenta begins when the blastocyst evokes the decidual reaction in the maternal endometrium (stage 5a) and transforms this into an exceedingly well perfused source of nutrition - the basal plate. The neighboring glands of the endometrium develop and the uterine wall gets to be turgid locally. If an implantation has taken place the trophoblast cells release a multiplicity of hormones (HCG = human chorionic gonadotrophine, HCS = human chorionic somatomammotrophine, and also HPL = human placental lactogen) that are necessary to coordinate the nourishment of the endometrium and to maintain the corpus luteum which, over roughly 12 weeks of the embryonic development, continues to secrete sexual steroids. Later, the placenta itself starts to secrete large amounts of progesterone and thereby takes over the glandular function of the corpus luteum.