Understanding the Menstrual Cycle, Fertilization, and Pregnancy

The Menstrual Cycle

Parallel to the changes that occur in the ovarian cycle, a series of cyclical changes take place in the uterine lining, or endometrium, which thickens its walls to prepare to host a fertilized egg. If no fertilization occurs, the endometrium is removed from the uterus and expelled through the vagina, along with the unfertilized egg, during menstruation.

Menstruation (also called menses or a period) is the periodic removal of the endometrium, which carries with it blood, the unfertilized egg, and other fluids.

The Female Reproductive Cycle

• Days 1–5: The cycle begins with menstruation. Simultaneously, the pituitary gland releases FSH, which stimulates the secretion of estrogen and initiates the growth of several follicles in one ovary. After several days, only one follicle reaches maturity, while the others degenerate.
• Day 14: The pituitary gland produces a massive secretion of FSH, causing the rupture of the mature follicle and triggering ovulation—the release of the egg from the follicle. The empty follicle becomes the corpus luteum (or yellow body), which releases progesterone.
• Post-Ovulation: In the following days, progesterone, along with estrogen, contributes to the progressive development of the endometrium to support a potential pregnancy. The uterine lining becomes soft and receptive, acting as a cradle for a developing zygote.
• Day 28: If the egg is not fertilized, it disintegrates. A sharp decrease in progesterone levels triggers a new period, shedding the endometrium and beginning a new reproductive cycle.

Fertilization and Pregnancy

Ovulation occurs fourteen days before the arrival of the next period. The egg released from the Graafian follicle is propelled along the fallopian tube toward the uterus, a journey taking about a week. The egg remains viable for fertilization for only 24 hours following ovulation.

The Process of Conception

Intercourse involves the introduction of the penis into the vagina. During ejaculation, sperm are released into the vagina. Unlike the egg, sperm are highly mobile; they ascend through the vagina, traverse the cervix and uterus, and reach the fallopian tubes within a few hours. Sperm can survive for three to five days, meaning sexual activity in the days leading up to ovulation carries a high probability of pregnancy.

If the egg and sperm meet, multiple sperm surround the egg, but only one penetrates the protective envelope. Once the sperm head enters, it sheds its tail. The genetic material of the sperm and egg then fuse to complete fertilization. The resulting cell is called a zygote.