Human chorionic gonadotropin is a heterodimer glycoprotein hormone, approximately 37 kDa in weight. There are two subunits of human chorionic gonadotropin: α-hCG and β-hCG. The α-subunit is identical to the α-subunits of luteinizing hormone, thyroid-stimulating hormone, and follicle-stimulating hormone. The β-subunit of hCG is unique. β-hCG is composed of 145 amino acids linked by six disulfide bridges. It has two N-linked and four O-linked oligosaccharides.

Multiple hCG-related molecules are present in pregnancy serum and urine samples. These include hCG in various stages of degradation, free α- and β-subunits, individual subunit fragments and hCG with altered glycosylation forms. Compared to normal pregnancies, much greater and variable proportions of hCG-related molecules are detected in Down’s syndrome, pre-eclampsia and trophoblastic disease. Additionally, there are temporal differences in the production of hCG variants. Early in pregnancy, a hyperglycosylated form of hCG (HhCG) is detected in serum and urine samples. This HhCG is phenotypically and antigenically similar to the hyperglycosylated hCG produced by gestational trophoblastic disease. Examination of urine pregnancy samples for total hCG and HhCG revealed that 68% of total hCG detected early in pregnancy is HhCG. By the end of the first trimester HhCG comprised less than 3% of total hCG. HhCG is produced by cytotrophoblast cells, which are mostly phenotypically invasive cells, while regular hCG is produced by differentiated syncytiotrophoblasts.

Initial detection of β-hCG is thought to coincide with implantation. Among pregnancies lasting greater than 6 weeks, implantation occurred on days 8, 9 and 10 after ovulation in 84% of women. Risk of early pregnancy loss increased with later implantation; 13%, 26%, 52% and 82% of pregnancies that implanted on day 9, 10, 11 and >11 resulted in early loss, respectively. In another study, hCG was detectable in the serum by day 10 postgonadotropin surge (9 days after ovulation) in 75% of pregnancies and this increased to >95% by day 11 postgonadotropin surge. If hCG was not detected in the serum until day 12, the pregnancy was more likely to result in miscarriage.

Serial measurements of hCG have been established as a useful tool in distinguishing viable from nonviable pregnancies. In the early 1980s, initial studies described the minimum increase in hCG levels over 48 hours in a normal intrauterine pregnancy to be 66%. More recently (in 2004), utilizing newer assay technologies and a larger cohort of patients, a 99% confidence interval for normal hCG rise was established. This report found the lowest rate of rise to be 53% over 48 hours, with a median rate of rise of 124% over 48 hours. Abnormal gestations are associated with slower doubling times, although a normal rate of rise does not rule out an abnormal gestation.

If the circulating hCG level is greater than 1500 mIU/mL and the transvaginal ultrasound notes no gestational sac or pole in the uterus, an ectopic gestation should be suspected. Transvaginal ultrasound studies performed with a 5 MHz probe suggest that a gestational sac should be evident with a serum hCG level of 1500 mIU/mL, approximately 23–29 days from the LMP; a yolk sac should be evident at 5000 mIU/mL, 32–45 days from the LMP; and fetal cardiac motion detected at 13,000–15,000 mIU/mL and 42 days from the LMP with a CRL >5 mm. A note of caution: a 4 mm sac looks similar to 4 mm of fluid. A false-positive sac may represent late follicular-phase endometrium, a pseudosac associated with an ectopic gestation or a venous lake in myometrium.

A useful rule of thumb is that hCG levels are approximately 100 mIU/mL at the time of missed menses (or 14 days after ovulation). Based on a 28-day cycle, production of hCG has been estimated to double within 1.4– 1.6 days up to the 35th day from the onset of the LMP and within 2.0–2.7 days from days 35–42. The level of hCG peaks at 8–10 weeks from the LMP at 10,000–80,000 mIU/mL and then drops and remains at approximately 10,000 mIU/mL for the remainder of pregnancy.