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)
