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Pregnancy Tests Detect hCG, a Hormone Made Only After Implantation
Definition
Human chorionic gonadotropin (hCG)
Human chorionic gonadotropin (hCG) is a hormone essential for sustaining early pregnancy. It ensures that the uterine environment remains suitable for the developing embryo by maintaining the production of progesterone.
- hCG is a hormone secreted by the trophoblast of the developing embryo, and later by the placenta.
- It maintains early pregnancy by signalling the corpus luteum to keep secreting progesterone.
- The trophoblast starts secreting hCG around 6 to 7 days after fertilisation, just before implantation.
- Because only the embryo and placenta make hCG shortly after fertilisation, its presence is a reliable early marker that implantation has occurred.
Note
- Without hCG, the corpus luteum would break down, causing menstruation and loss of the embryo.
- See D3.1.16 for how the blastocyst implants and starts secreting hCG.
Monoclonal Antibodies Bind hCG to Give a Visible Line
- Pregnancy tests detect hCG in urine using monoclonal antibodies.
- Monoclonal antibodies are identical antibodies, specific to a single antigen, made by one clone of cells.
Definition
Monoclonal antibodies
Monoclonal antibodies are lab-engineered antibodies designed to bind to a specific target—in this case, hCG.
Tip
- Antibodies are proteins that recognise and bind to specific molecules called antigens.
The Three Zones of a Test Stick
- Reaction zone
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- D3.1.17 Pregnancy testing by detection of human chorionic gonadotropin secretion (HL)
- D3.1.18 Role of the placenta in foetal development inside the uterus (HL)
- D3.1.19 Hormonal control of pregnancy and childbirth (HL)
- D3.1.20 Hormone replacement therapy and the risk of coronary heart disease (HL)