Q.Corpus luteum in pregnancy has a long life. However, if fertilisation does not take place, it remains active only for 10-12 days. Explain.

If fertilisation does not occur, the corpus luteum degenerates after approximately 10-12 days. This is because there is no signal to sustain it, leading to a sharp decline in progesterone secretion. The drop in progesterone causes the disintegration of the uterine endometrium, resulting in menstruation.

However, if fertilisation takes place, the developing embryo begins to secrete human chorionic gonadotropin (hCG) shortly after implantation. This hCG acts to maintain the corpus luteum, preventing its degeneration. The sustained corpus luteum continues to secrete large amounts of progesterone, which is crucial for maintaining the uterine lining and preventing uterine contractions, thereby supporting the early stages of pregnancy until …

To understand why the corpus luteum has a varying lifespan depending on whether fertilisation occurs, we must first appreciate its critical role in the female reproductive cycle and early pregnancy. The corpus luteum is a temporary endocrine structure that forms in the ovary after ovulation. Following the release of the ovum from the Graafian follicle, the remaining follicular cells transform into the corpus luteum under the influence of Luteinizing Hormone (LH). Its primary function is to secrete large amounts of progesterone, along with some estrogen.

Progesterone is vital for preparing and maintaining the uterine endometrium, making it suitable for implantation of a fertilised egg and supporting the early stages of pregnancy. It thickens the uterine lining, increases its blood supply, and reduces uterine contractions, all essential conditions for a successful pregnancy.

Scenario 1: If Fertilisation Does Not Take Place

If the released ovum is not fertilised, or if fertilisation occurs but implantation fails, the corpus luteum has a limited lifespan, typically around 10-12 days. In the absence of a signal indicating pregnancy, the levels of Luteinizing Hormone (LH) from the pituitary gland, which initially stimulated its formation and function, begin to decline. Without sustained LH support, the corpus luteum starts to degenerate.

As the corpus luteum degenerates, its production of progesterone and estrogen sharply decreases. This sudden drop in hormone levels destabilises the thickened uterine endometrium, leading to its breakdown and shedding. This shedding of the uterine lining, accompanied by blood, is what we know as menstruation, marking the beginning of a new menstrual cycle. The short lifespan of the corpus luteum in this scenario ensures that the uterus is cleared and prepared for a potential new cycle and ovulation.

Scenario 2: If Fertilisation Takes Place and Pregnancy Occurs

When fertilisation successfully occurs, and the resulting embryo implants in the uterine wall, a different sequence of events unfolds, extending the life of the corpus luteum significantly. Shortly after implantation, the developing embryo begins to secrete a crucial hormone called human chorionic gonadotropin (hCG).