Placenta accreta is the condition where the embryo fully burrows through the endometrium and adheres to the myometrium, resulting in potentially life-threatening postpartum hemorrhage. This condition is specifically associated with the uterus. The invasive nature of the human embryo during ectopic implantation poses significant risks to the mother. Ectopic implantation refers to the embryo implanting at a site other than the uterus, leading to an ectopic pregnancy. The majority of ectopic pregnancies (over 90%) occur within the oviducts, also known as tubal pregnancies. However, they can also occur in the ovary and abdominal cavity. Implantation in the oviduct often leads to long-term infection and inflammation, known as pelvic inflammatory disease, as well as obstruction of the tube. In tubal pregnancies, the highly invasive nature of the human syncytiotrophoblast, which is usually controlled by the uterine decidual response, typically results in the embryo burrowing through the oviduct wall. While abdominal pregnancies can reach full term, undetected oviductal pregnancies usually lead to rupture of the oviductal wall. The resulting internal hemorrhage can be catastrophic and requires immediate surgical intervention.

The first hormone produced by syncytiotrophoblasts is hCG, which is structurally related to the pituitary glycoprotein hormones. hCG consists of a common α-glycoprotein subunit (α-GSU) and a hormone-specific β subunit (β-hCG). Antibodies used to detect hCG in laboratory assays and pregnancy tests are designed to specifically detect the β subunit. hCG bears the greatest similarity to LH and has a high affinity for the LH receptor. The β subunit of hCG is longer than that of LH and contains more glycosylation sites, significantly increasing its half-life to 24 to 30 hours. The stability of hCG allows it to accumulate rapidly in the maternal circulation, making it detectable within 24 hours of implantation. Serum hCG levels double every 2 days during the first 6 weeks and peak at approximately 10 weeks. Subsequently, serum hCG levels decline to a constant level at around 50% of the peak value (Fig. 44.34A •Fig. 44.34 Circulating levels of hCG and hPL in maternal blood during pregnancy. (Modified from White BA, Porterfield SP. Endocrine Physiology. 4th ed. Philadelphia: Mosby;2013.)100 80 60Human chorionic gonadotropin(mIU/mL)40 20 10 0 10 20 30 40 Weeks of gestationA B 10 8Human placental lactogen(µg/mL)6 4 2 0 10 20 30 40 Weeks of gestation
The main function of hCG is to stimulate LH receptors on the corpus luteum, preventing luteolysis and maintaining a high level of progesterone production derived from the corpus luteum during the first 10 weeks of pregnancy. The rapid increase in hCG is responsible for the nausea commonly known as "morning sickness" in early pregnancy. A small percentage (1%–10%) of hCG enters the fetal circulation. hCG stimulates fetal Leydig cells to produce testosterone before the fetal gonadotropic axis is fully developed. Additionally, during the first trimester, hCG stimulates the fetal adrenal cortex (as discussed later).
Placenta accreta istheburrowingoftheembryocompletelythroughtheendometriumandadhesionoftheplacentatothemyometrium,aconditionassociatedwithpotentiallylife-threateningpostpartum hemorrhage. Thedecidualresponseoccursonlyintheuterus.Thusthehighlyinvasivenatureofthehumanembryoposesconsiderablerisktothemotherinthecaseofectopic implantation. Ectopic implantation referstoimplantationofanembryoatasiteotherthantheuterus,andectopic pregnancy referstoadevelopingembryoatasiteofectopicimplantation.Mostectopicpregnancies(>90%)occurwithintheoviducts(calledtubal pregnancies),buttheycanalsooccurintheovaryandabdominalcavity.Implantationintheoviductsisoftenassociatedwithlong-terminfectionandinflammation(calledpelvic inflammatory disease)andobstructionofthetube.Inatubalpregnancythehighlyinvasivenatureofthehumansyncytiotrophoblast,whichisnormallymoderatedbytheuterinedecidualresponse,usuallyleadstoburrowingoftheimplantedembryothroughthewalloftheoviduct.Althoughabdominalpregnanciescanproceedtoterm,undetectedoviductalpregnanciesusuallyleadtoruptureoftheoviductalwall.Theresultinginternalhemorrhagecanbecatastrophictothemotherandrequiresimmediatesurgicalintervention.
The first hormone produced by syncytiotrophoblasts is hCG, which is structurally related to the pituitary glycoprotein hormones (see ). As such, hCG is composed of a common α-glycoprotein subunit (α-GSU) and a hormone-specific β subunit (β-hCG). Antibodies used to detect hCG (i.e., in laboratory assays and over-the-counter pregnancy tests) are designed to specifically detect the β subunit. hCG is most similar to LH and binds with high affinity to the LH receptor. The β subunit of hCG is longer than that of LH and contains more sites for glycosylation, which greatly increases the half-life of hCG to 24 to 30 hours. The stability of hCG allows it to rapidly accumulate in the maternal circulation such that hCG is detectable within maternal serum within 24 hours of implantation. Serum hCG levels double every 2 days for the first 6 weeks and peak at about 10 weeks. Serum hCG then declines to a constant level at about 50% of the peak value (Fig. 44.34A •Fig. 44.34 CirculatinglevelsofhCGandhPLinmaternalbloodduringpregnancy.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.)100 80 60Human chorionic gonadotropin(mIU/mL)40 20 10 0 10 20 30 40 Weeks of gestationA B 10 8Human placental lactogen(µg/mL)6 4 2 0 10 20 30 40 Weeks of gestation
The primary action of hCG is to stimulate LH receptors on the corpus luteum. This prevents luteolysis and maintains a high level of luteal-derived progesterone production during the first 10 weeks. The rapid increase in hCG is responsible for the nausea of “morning sickness” associated with early pregnancy. A small amount (i.e., 1%–10%) of hCG enters the fetal circulation. hCG stimulates fetal Leydig cells to produce testosterone before the fetal gonadotropic axis is fully mature. hCG also stimulates the fetal adrenal cortex (see later) during the first trimester.