The blastocyst usually implants on the posterior uterine wall. The implantation begins at the end of the first week and is completed by the end of the second week. During the 6th day, the blastocyst attaches to the endometrial epithelium. During the second week, the trophoblast begins to differentiate into the syncytiotrophoblast and the cytotrophoblast. Penetration of the blastocyst into the mucosa result from proteolitic enzymes produced by the syncytiotrophoblast. At the beginning of the second week the blastocyst is embedded in the endometrial stroma. The endometrial cells around the early conceptus enlarge and accumulate glycogen and lipids. These cellular changes, together with the vascular and glandular alterations in the endometrium, are called the decidual reaction.

1. Endometrium
2. Uterine wall
3. Implanted blastocyst
4. Uterine tube














1. Blood vessel
2. Endometrial stroma
3. Syncytiotrophoblast
4. Cytotrophoblast
5. Surface epithelium
6. Epiblast
7. Aminotic cavity
8. Hypoblast

Three different regions of the decidua are identified according to the implantation site. The decidua basalis is the portion of the endometrium that underlies the implantation site. The decidua basalis forms a compact layer, called the decidual (basal) plate. The decidua capsularis is a thin portion of the endometrium that overlies the conceptus. The decidua parietalis (vera) includes the remaining endometrium of the uterus and the cervix.