Question

Why are menstrual cycles absent during pregnancy?

Answer 1

Menstrual cycle

The reproductive cycle in female primates is known as the menstrual cycle. In humans, the cycle repeats after every $28$ days. It consists of $4$ phases: menstrual phase, follicular phase, ovulatory phase and luteal/ secretory phase.

Hormonal control of menstrual cycle and pregnancy

Gonadotropin releasing hormones from the hypothalamus stimulates the pituitary to release $FSH$ and $LH$ which regulate the events of the menstrual cycle. At the onset of the cycle, the levels of $FSH$ and $LH$ remain low and increase gradually.

$FSH$ stimulates the development of the graafian follicles during the follicular phase of the cycle. The mature graafian follicles release estrogen.

As the level of estrogens and $LH$ increases during the $14th$ day (ovulatory phase) of the cycle, the graafian follicle ruptures to release the ovum. The graafian follicles after the release of the ovum changes to corpus luteum. This marks the onset of the luteal phase and the levels of $LH$ and $FSH$ drop during this phase.

In absence of fertilisation, the corpus luteum degenerates. However, in the instance of conception, the corpus luteum is maintained and it continues to secrete progesterone. These prepare the uterine wall for implantation.

Absence of menstrual cycle during pregnancy

The onset of the next menstrual cycle occurs when the level of estrogens and progesterone is low due to the degeneration of corpus luteum, in the absence of fertilisation. This causes the endometrium lining to shed, marking the beginning of the menstrual phase of the next cycle. However, during pregnancy, the level of both these hormones is high due to which the endometrium continues to thicken to make it suitable for implantation. Hence, menstrual cycles remain absent for the entire gestation or pregnancy period.