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.