Human Chorionic Gonadotropin (hCG): The First Placental Hormone
The answer is A. Human chorionic gonadotropin (hCG) is the first placental hormone produced during pregnancy.
Understanding hCG Production
Human chorionic gonadotropin (hCG) is produced by trophoblastic cells and is the earliest hormonal message from the developing placenta to the mother 1. It is detectable in maternal blood just two days after implantation 2, 3, making it the first hormone produced by the placental tissue.
Key characteristics of hCG:
- Produced by trophoblastic cells of the developing placenta 1
- Detectable in maternal blood at very early stages of pregnancy
- Serum values exceeding 5 mIU/mL indicate pregnancy 1, 4
- Considered the first biochemical evidence of pregnancy
Structure and Function of hCG
hCG is a complex glycoprotein composed of two highly glycosylated subunits:
- Alpha subunit: Identical to other pituitary gonadotropin hormones (LH, FSH, TSH)
- Beta subunit: Unique to hCG, contains two N-glycosylation sites and four O-glycosylation sites 2, 3
Primary Functions:
- Acts as a super LH agonist to stimulate progesterone secretion by the corpus luteum 2
- Maintains progesterone production until the placenta takes over this function
- Promotes formation of the syncytiotrophoblast 3
- Supports myometrial quiescence and local immune tolerance 2
- Plays crucial roles in trophoblast invasion and angiogenesis 3
Different Forms of hCG
hCG exists in multiple forms with varying functions:
- Regular hCG: Primarily secreted by syncytiotrophoblast cells
- Hyperglycosylated hCG (hCG-H): Secreted by invasive extravillous trophoblast cells
- Free beta-subunit of hCG: Can be produced by certain malignancies 3, 5
Timing and Levels During Pregnancy
hCG levels follow a characteristic pattern during pregnancy:
- First detectable in maternal blood two days after implantation 2, 3
- Levels rise rapidly in early pregnancy
- Peak at approximately 8-10 weeks gestation 4, 3
- Begin to decline after 10-11 weeks 4
- Continue to gradually decrease throughout the second and third trimesters 4
Clinical Applications
hCG measurement has numerous clinical applications:
- Confirmation of pregnancy (serum hCG >5 mIU/mL) 1
- Early pregnancy monitoring 4
- First-trimester screening for chromosomal abnormalities when combined with other markers 1, 4
- Monitoring gestational trophoblastic disease 1
- Assessment of pregnancy viability and potential complications 4
Important Considerations in hCG Testing
When interpreting hCG results, clinicians should be aware that:
- Different laboratories may use different assays and reference ranges 4
- The beta modifier may no longer be used by some laboratories 1
- Rarely, elevated hCG may be unrelated to pregnancy (e.g., pituitary dysfunction or neoplasia) 1
- hCG values should not be used in isolation for clinical decision-making beyond early pregnancy 4
In summary, human chorionic gonadotropin (hCG) is definitively the first placental hormone produced during pregnancy, making option A the correct answer.