After fertilization the blastocyst will implant onto the endometrium with interaction between fetal trophoblasts and maternal endometrium-decidua. Implantation occurs 8 to 10 days after fertilization, when the conceptus is a blastocyst.


Within 24 hours after implantation, or at about 3 weeks of gestation, human chorionic gonadotropin (hCG), produced by syncytiotrophoblasts, is detectable in maternal serum. Under the influence of increasing hCG production, the corpus luteum secretes progesterone and estradiol in increasing quantities.

Vitronectin, an alpha-v-beta-3 integrin receptor ligand, serves as one of several links between the maternal and embryonic epithelia. The two layers of placental epithelial cells, cytotrophoblasts and syncytiotrophoblasts, develop after the blastocyst invades the endometrium.

Image not available.

Microanatomy of the human placental bed. Oxygen, nutrients, and waste products are exchanged between the fetus and the mother across this interface. In normal placental development (upper panel), invasive cytotrophoblasts of fetal origin invade the maternal spiral arteries, displacing maternal endothelial cells and transforming the vessels from small-caliber resistance vessels to high-caliber capacitance vessels capable of placental perfusion adequate to sustain fetal growth. In preeclampsia and some cases of fetal growth restriction, cytotrophoblasts fail to adopt an invasive endothelial phenotype. Instead, invasion of the spiral arteries is shallow and they remain small-caliber resistance vessels (lower panel). This can lead to placental ischemia.

(Adapted, with permission, from Karumanchi SA, et al. Preeclampsia: a renal perspective. Kidney Int. 2005;67:2101.)


Columns of invading cytotrophoblasts anchor the placenta to the endometrium. The differentiated syncytiotrophoblast, derived from fusion of cytotrophoblasts, is in direct contact with the maternal circulation. The syncytiotrophoblast is the major source of hormone production, containing the cellular machinery needed for synthesis and secretion of both steroid and polypeptide hormones.





The ability of mother and fetus to coexist as two distinct immunological systems results from endocrine, paracrine, and immunological modification of fetal and maternal tissues. The placenta mediates a unique fetal–maternal communication system, which creates a hormonal environment that initially maintains pregnancy and eventually initiates events leading to parturition.


Implantation of the blastocyst into the endometrium usually occurs on which days after fertilization?

The correct answer is C.

C. The blastocyst typically enters the endometrial cavity 6 days after fertilization, and implantation occurs near the seventh day. During implantation, patients may have a "bloody vaginal discharge" or a brief episode of vaginal spotting that may be mistaken for a "light" period.


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