The menstrual cycle is a natural, recurring process in the female body, preparing for the possibility of pregnancy each month. This biological rhythm involves hormonal changes and physical transformations within the reproductive system. A central aspect of this cycle involves the egg, or ovum, and its journey.
The Egg’s Path to Ovulation
The journey of an egg begins within fluid-filled sacs in the ovaries called follicles. Each month, several follicles develop, stimulated by hormones like follicle-stimulating hormone (FSH) from the pituitary gland. Typically, only one follicle becomes dominant and continues to mature. As this dominant follicle grows, it nourishes the developing egg inside.
Around day 10 to 14 of an average 28-day cycle, this maturing follicle produces increasing amounts of estrogen, which signals a surge in luteinizing hormone (LH). This LH surge triggers the release of the egg from the ovary, an event known as ovulation. Once released, the egg is swept by finger-like projections called fimbriae into the fallopian tube, where it begins its journey towards the uterus.
What Happens When Fertilization Doesn’t Occur
After ovulation, the released egg has a limited window of viability for fertilization, typically lasting only 12 to 24 hours. If sperm are not present in the fallopian tube, or if fertilization fails, the egg quickly begins to degenerate. It ceases to be viable, and its cellular structures start to break down.
This process of degradation means the egg does not simply vanish. Instead, it disintegrates and is then reabsorbed by the body. The unfertilized egg does not survive to be expelled as a whole entity during menstruation, but rather its components are broken down and reprocessed.
The Menstrual Cycle and Expulsion
The lining of the uterus, known as the endometrium, thickens throughout the menstrual cycle. This thickening is influenced by hormones like estrogen and progesterone, creating a nutrient-rich environment for a fertilized egg to implant. If fertilization and subsequent implantation do not occur, the hormonal balance shifts.
Levels of progesterone and estrogen, which were maintaining the thickened uterine lining, drop sharply. This decline signals that pregnancy has not happened, leading to the shedding of the uterine lining. This shedding process constitutes menstruation, commonly known as a period. The menstrual flow consists of blood, tissue, and mucus from the endometrial lining. The disintegrated remnants of the unfertilized egg are expelled along with this material, completing the cycle.