hCG physiology

• hCG is produced during pregnancy, primarily by the placental syncytiotrophoblast.
• The role of hCG is to:
  • Stimulate progesterone secretion by the corpus luteum during the first 3–6 weeks of gestation to maintain pregnancy
    • Prior to pregnancy, LH stimulates corpus luteal progesterone secretion.
    • After the luteal placental shift, the placenta produces progesterone.
      • The time period around weeks 6-7 of gestation, in which the function of the corpus luteum declines and the production of progesterone gradually shifts from the corpus luteum to the placenta.
  • Promote uterine angiogenesis
  • Promote myometrial stability, preventing contractions prior to labor
  • Support immune tolerance to the growing embryo
• Structure
  • α-subunit: common to hCG, FSH, LH, and TSH
  • β-subunit
    • Specific to hCG
    • Pregnancy tests generally detect hCG through antibodies to the β-subunit.

Types of pregnancy tests

• hCG begins being produced by syncytiotrophoblast after invasion of endometrial connective tissue 6-7 days after fertilization
• Serum level at 8 days is <5 IU/L, once serum is 20 IU/L this can be detected in a urine pregnancy test (~14 days following fertilization)