The Menstrual Cycle: The stages of a normal cycle

The Menstrual Cycle, spanning around 28 days, is a rhythmic process that orchestrates hormonal shifts and physiological changes. From the onset of menstruation to the pinnacle of ovulation and the luteal phase, each stage plays a crucial role in fertility and reproductive well-being.

Egg Formation and Maturation: Women (assigned female at birth) are born with all of the eggs that they will ever have. These eggs are created during fetal development in follicles (fluid-filled sacs) inside the fetal ovaries. These eggs are immature and incapable of being fertilized with sperm. Though these eggs begin to mature in the fetal ovaries, their development is halted early on and remains halted until puberty.

The Follicular Phase: At the onset of puberty, a woman will begin having monthly menstrual cycles, which typically last around 28 days (though variations are common). Each cycle, the brain produces hormones (particularly follicle stimulating hormone, or FSH) that cause a cohort (group) of immature eggs to resume their development (maturation). Egg maturation causes the ovaries to release increasing amounts of the hormone estrogen (E2), which travels to the uterus to help its lining (endometrium) thicken in preparation for embryo implantation, and also to the brain.

Ovulation: Though multiple eggs begin to mature, only one (sometimes two) egg ultimately matures while the others degenerate. Once the E2 levels reach a certain amount, the brain releases a surge of luteinizing hormone (LH), which triggers the mature eggs to be released from its follicle in the ovary in a process known as ovulation. The egg is quickly picked up by fingerlike projections (fimbriae) from the nearby fallopian tube.

The Luteal Phase: The egg spends the next few days traveling down the fallopian tube toward the uterus. At this time, sperm can enter the female reproductive tract and fertilize the egg inside the fallopian tube. Also, the follicle that once contained the egg transforms into a structure known as the corpus luteum (“yellow body”), which remains in the ovary and produces the hormones progesterone (P4) and E2. These hormones travel to the uterus to help prepare the endometrium for embryo implantation.

End of Cycle: After a few days, the egg (fertilized or unfertilized) enters the uterus. If fertilization occurred, it is now called an embryo. An embryo has the ability to implant into the endometrium to initiate a pregnancy. If this occurs, the corpus luteum in the ovary will continue to produce P4 for about 8-10 weeks, at which point the placenta will take over P4 production to sustain the pregnancy.

If fertilization does not occur or the embryo does not implant, the corpus luteum will regress, and the uterine lining will break down and shed through the cervix and vagina (i.e. the woman will get her period). If this occurs, the menstrual cycle will repeat itself.