Clinical Pathology: General Principles, Lab Management, Clinical Chemistry

• Human chorionic gonadotropin (hCG) is a glycoprotein consisting of an α- and β-subunit. The α-subunit is identical to the α-subunits of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). hCG is produced by placental syncytiotrophoblastic cells and is used as a marker for pregnancy detection and monitoring of pregnancy. hCG is also produced by trophoblast cells in hydatidiform moles and choriocarcinoma and in patients with germ cell carcinomas. The pituitary gland is also a source of hCG.

• In a normal pregnancy, hCG levels rise exponentially, doubling about every 2 days for about 8 weeks after implantation of the embryo. Maximum levels are reached at 8 to 10 weeks after the last menstrual period, and hCG levels higher than 100,000 IU/L can be obtained.

• After the first trimester, hCG levels decrease and remain at a constant level during the third trimester. A wide range of hCG values can be obtained during pregnancy. Serum hCG levels can take up to 4 weeks to return to baseline after a normal delivery.

• In an ectopic pregnancy, lower-than-expected serum hCG values are obtained compared with a normal pregnancy. In addition, serum hCG levels increase at a slower rate than in a normal pregnancy and do not double every 2 days. Low serum hCG levels and a lack of doubling every 2 days are suggestive of an ectopic pregnancy.

• Hydatidiform moles occur in approximately 1 in 1500 to 2000 pregnancies in the United States. Patients with hydatidiform moles present with a uterus larger than expected for gestational age, absence of fetal heart tones, dark brown to bright red vaginal discharge in the fourth or fifth month of pregnancy, expulsion of cystic vesicles, markedly elevated serum hCG levels, severe nausea and vomiting during pregnancy, and pregnancy-induced hypertension before 24 weeks of gestation. Patients with a complete hydatidiform mole can have serum hCG levels higher than 1,000,000 IU/L.