The human reproductive system serves four core functions: producing egg and sperm cells, transporting and sustaining those cells, nurturing a developing offspring, and producing hormones that regulate everything from puberty to bone health. While reproduction is the obvious purpose, the hormones this system generates influence your body far beyond fertility.
Producing Eggs and Sperm
The primary reproductive organs, the ovaries and testes, are responsible for creating the sex cells (gametes) that make new life possible. How they do this differs dramatically between males and females.
The testes produce between 100 and 300 million sperm every single day. Each sperm cell takes about 70 days to develop fully. After forming in the testes, immature sperm travel to a coiled tube called the epididymis, where they spend roughly 12 days gaining the ability to swim. They’re then stored there until ejaculation, when a single release contains 200 to 300 million sperm. Despite those numbers, only one will fertilize an egg.
The ovaries work on a completely different scale. A female is born with all the eggs she will ever have, typically around one million at birth. By puberty, that number drops to roughly 500,000, and each month after puberty, up to 1,000 eggs are lost. Of those, only one mature egg is released per menstrual cycle. Over an entire reproductive lifespan, roughly 480 eggs will actually be ovulated.
Transporting and Protecting Sex Cells
Creating gametes is only the first step. The reproductive system also moves them to the right place and keeps them alive long enough to meet.
In males, a series of accessory glands add protective fluids as sperm travel from storage to ejaculation. The seminal vesicles contribute fructose, a sugar that fuels sperm motility. The prostate adds an alkaline fluid that thickens semen, helping sperm survive the acidic environment inside the female reproductive tract. The bulbourethral glands secrete a thick lubricant that clears residual urine from the urethra before sperm pass through. Healthy semen contains at least 15 million sperm per milliliter, and at least 40% of those sperm need to be moving well for pregnancy to occur.
In females, finger-like structures called fimbriae sweep the released egg from the ovary into the fallopian tube and guide it toward the uterus. Fertilization typically happens during this journey through the tube, within 24 hours of ovulation.
Fertilization and Early Development
When sperm reach the egg in the fallopian tube, millions compete but only one breaks through the egg’s outer layer. The moment that happens, the fertilized egg (now called a zygote) begins dividing: two cells, then four, then more. About six days after fertilization, it has become a cluster of roughly 100 cells called a blastocyst. This blastocyst then attaches to the thickened lining of the uterus in a process called implantation, which marks the true beginning of pregnancy.
The uterine lining has been preparing for this moment all cycle long, thickening under the influence of hormones so it can receive and nourish the embryo. If no fertilized egg implants, that lining sheds during menstruation and the cycle starts over.
The Menstrual Cycle
The menstrual cycle is the reproductive system’s monthly preparation for a potential pregnancy, and it runs on a tightly coordinated sequence of hormonal signals. It has two main phases, with ovulation as the dividing event.
During the first phase, called the follicular phase, rising levels of follicle-stimulating hormone (FSH) prompt a group of follicles in the ovary to begin developing. One follicle becomes dominant and produces increasing amounts of estrogen, which thickens the uterine lining. As estrogen climbs, it triggers a surge of luteinizing hormone (LH). About 10 to 12 hours after that LH peak, ovulation occurs and the mature egg is released.
The second phase, the luteal phase, lasts about 14 days. The empty follicle transforms into a structure that pumps out progesterone and estrogen, maintaining the uterine lining. Progesterone and estrogen peak around eight or nine days after ovulation. If pregnancy doesn’t occur, hormone levels drop, the lining breaks down, and menstruation begins, restarting the cycle.
Hormones Beyond Reproduction
The reproductive system is also an endocrine powerhouse. The hormones it produces shape your body from before birth through old age, and their influence extends well beyond fertility.
Testosterone drives the development of male sex organs in the fetus. During puberty, it deepens the voice, stimulates facial and body hair growth, triggers growth spurts, and builds muscle mass and bone strength. Throughout adulthood, it remains essential for sperm production and maintaining muscle and bone. Testosterone also has strong protein-building effects, which is why it plays such a central role in body composition.
Estrogen orchestrates the development and maintenance of the female reproductive organs and breasts. During puberty, it shapes the pattern of fat distribution that creates a typical female body contour, regulates growth, and drives development of secondary sexual characteristics. In adult women, estrogen regulates the menstrual cycle, supports pregnancy and breastfeeding, and helps maintain libido. Men also need estrogen in smaller amounts for bone maintenance and brain function.
Both hormones contribute to bone density throughout life. When their levels decline with age, bone loss accelerates, which is one reason osteoporosis risk rises after menopause.
How the System Changes With Age
Reproductive aging looks quite different in males and females. In women, menopause occurs at an average age of around 46, marked by at least 12 consecutive months without a period. The ovaries stop releasing eggs and estrogen production drops sharply. This decline triggers a cascade of changes: hot flashes (driven partly by shifts in how the brain’s temperature-regulation center responds), bone loss, and changes in cardiovascular risk. Every woman goes through menopause.
In men, the process is far more gradual and variable. Testosterone levels decline progressively over time as the cells that produce it slowly degenerate. However, most men’s bodies compensate by increasing LH production, which stimulates the testes to maintain adequate testosterone. Only about 20% of men over 65 have testosterone levels that drop below the normal range for younger men. When levels do fall significantly, it can lead to reduced muscle mass, lower energy, decreased bone density, and changes in sexual function.