D3.1.17 Pregnancy testing by detection of human chorionic gonadotropin secretion (HL) Notes

Pregnancy Tests Rely On The Detection of Human Chorionic Gonadotropin (hCG)

1. hCG is a hormone produced by the developing embryo and placenta.
2. It is essential for maintaining early pregnancy by stimulating the corpus luteum to secrete progesterone.

How is hCG produced?

1. Fertilization and Early Development
   1. After fertilization, the zygote divides and forms a structure called the blastocyst.
   2. The outer layer of the blastocyst, called the trophoblast, eventually develops into the placenta.
2. Role of the Placenta
   1. The trophoblast begins secreting hCG around 6–7 days after fertilization, just before implantation.
   2. hCG signals the corpus luteum in the ovary to continue producing progesterone, preventing the uterine lining from breaking down.

Why is hCG Important?

1. Maintains the Uterine Lining: hCG ensures the endometrium remains thick and nutrient-rich, supporting the embryo.
2. Supports Placental Development: hCG helps the placenta grow and function effectively.

How Pregnancy Tests Work

1. Pregnancy tests detect hCG in urine using monoclonal antibodies.
2. Antibodies that are identical and specific to a single antigen, produced by a single clone of cells.
3. hCG is specifically used because it is uniquely produced by the developing embryo and placenta shortly after fertilization, making it an early and reliable marker of pregnancy.
4. Its presence in urine or blood provides a clear signal that implantation has occurred.

The Structure of a Pregnancy Test

A typical pregnancy test stick has three key zones:

1. Reaction Zone
   1. Contains free monoclonal antibodies that bind to hCG.
   2. These antibodies are linked to a colored dye.
2. Test Zone
   1. Contains immobilized antibodies that capture the hCG-antibody-dye complex.
   2. If hCG is present, a colored line appears.
3. Control Zone
   1. Contains antibodies that bind to the free antibodies (whether or not they are bound to hCG).
   2. Ensures the test is working correctly by displaying a second line.

Step-by-Step Process of Detection

1. Urine Application: The user applies urine to the test stick.
2. Binding in the Reaction Zone: If hCG is present, it binds to the free monoclonal antibodies.
3. Movement Along the Test Strip: The hCG-antibody-dye complex moves along the strip by capillary action.
4. Detection in the Test Zone: The complex is captured by immobilized antibodies, forming a visible colored line.
5. Validation in the Control Zone: Free antibodies bind in the control zone, forming a second line to confirm the test is working correctly.

Why Pregnancy Tests are Effective

1. Sensitivity: Modern tests can detect hCG as early as 10 days after fertilization.
2. Specificity: Monoclonal antibodies are highly specific to hCG, minimizing false positives.

Limitations of Pregnancy Tests

1. Timing: Testing too early may result in a false negative if hCG levels are too low.
2. False Positives: Rarely, other conditions (e.g., certain tumors) can produce hCG.
3. User Error: Incorrect use can lead to inaccurate results.

Reflection

1. Pregnancy tests are a remarkable example of how biology and technology intersect to provide critical information.
2. By understanding the role of hCG and the science behind monoclonal antibodies, you gain insight into the precision and reliability of these tests.