The female reproductive system serves several interconnected functions: producing eggs, enabling fertilization, supporting pregnancy, delivering a baby, and producing hormones that influence health well beyond reproduction. These functions operate across a reproductive lifespan that typically begins with the first menstrual period around age 13 to 16 and continues until menopause, which occurs around age 49 to 52.
Egg Production and the Ovarian Reserve
The ovaries are the system’s primary organs, and their most fundamental job is producing and storing eggs (oocytes). Unlike sperm, which are continuously generated, eggs are finite. During fetal development, the ovaries may contain a few million immature eggs. By birth, that number drops to roughly 600,000. By puberty, about half remain, and by age 20, roughly one-third of the original supply is left. This steady decline continues until the pool reaches nearly zero around age 50, which is when most women enter menopause.
Of those hundreds of thousands of eggs, only about 300 to 400 will ever be released during ovulation over a lifetime. That means a woman is born with over a thousand times more eggs than she will ever use. The rest are gradually reabsorbed by the body in a natural process called follicular atresia.
Hormonal Regulation
The entire system runs on a hormonal feedback loop between the brain and the ovaries. A region of the brain called the hypothalamus releases a signaling hormone in pulses. This prompts the pituitary gland, a small structure at the base of the brain, to release two key hormones: one that stimulates egg-containing follicles to grow and mature (FSH), and one that triggers ovulation and supports the structures left behind afterward (LH).
The ovaries respond by producing estrogen and progesterone, which feed information back to the brain. When estrogen and progesterone levels are high enough, the brain scales back its signals. When levels drop, the brain ramps up production again. This loop keeps the menstrual cycle running on a roughly monthly rhythm, and disruptions at any point in the chain can affect ovulation, periods, and fertility.
The Menstrual Cycle
A normal menstrual cycle lasts anywhere from 21 to 35 days, with bleeding lasting 2 to 7 days. The cycle has two main phases, each driven by different hormones and serving a different purpose.
The first phase, called the follicular phase, starts on the first day of your period. During this time, rising hormone levels cause several follicles in the ovaries to begin maturing. Usually one follicle becomes dominant and continues developing while the others break down. Simultaneously, the lining of the uterus (the endometrium) thickens and develops a richer blood supply, essentially building a potential landing site for a fertilized egg.
Ovulation marks the transition between phases. A surge in LH causes the dominant follicle to release its egg. What remains of that follicle transforms into a temporary hormone-producing structure called the corpus luteum, which secretes progesterone. This begins the luteal phase, during which progesterone further prepares the uterine lining for implantation. If no fertilized egg arrives, the corpus luteum breaks down, progesterone drops, and the lining sheds as a menstrual period, restarting the cycle.
Transporting and Fertilizing the Egg
Once an egg is released from the ovary, it doesn’t travel on its own. Finger-like structures called fimbriae at the ends of the fallopian tubes sweep the egg inward. From there, tiny hair-like projections lining the tubes, along with rhythmic muscle contractions, move the egg toward the uterus. Fertilization typically happens inside the fallopian tube, where sperm and egg meet. If fertilization occurs, those same cilia and muscle contractions guide the resulting embryo down into the uterus for implantation.
Supporting Pregnancy
Once a fertilized egg implants in the uterine lining, the system shifts into pregnancy maintenance. Early on, the developing embryo sends a hormonal signal (hCG, the hormone detected by pregnancy tests) that keeps the corpus luteum alive and producing progesterone. This prevents the uterine lining from shedding.
By about 6 to 10 weeks, the placenta takes over as the primary hormone factory. It produces progesterone, estrogen, and several other hormones that collectively reshape the body to support the growing fetus. Progesterone keeps the uterus relaxed by counteracting signals that would otherwise cause contractions. It also promotes weight gain and fat storage to build energy reserves. Estrogen supports blood vessel development in the uterus and helps regulate blood flow to the placenta. Another placental hormone adjusts the way the body processes sugar, ensuring a steady supply of glucose and amino acids reaches the fetus even if the mother hasn’t eaten recently. This is why pregnancy can shift your metabolism noticeably, sometimes leading to gestational changes in blood sugar regulation.
The placenta also produces a growth hormone variant that helps regulate fetal growth. Low levels of this hormone are linked to restricted fetal growth, which highlights how the placenta’s endocrine activity directly shapes outcomes for the baby.
Built-In Infection Defense
The reproductive system also protects itself. The vagina maintains a naturally acidic environment, with a typical pH of around 4.0 to 4.9. This acidity comes from beneficial bacteria, primarily from the Lactobacillus genus, that break down glycogen in vaginal tissue to produce lactic acid. This low pH creates a hostile environment for many sexually transmitted pathogens and opportunistic infections.
When Lactobacillus populations decline and pH rises above 4.5, the risk of bacterial vaginosis increases, along with elevated risks of infertility, preterm birth, maternal infections, and sexually transmitted diseases. This microbial defense system is one reason that douching and other practices that disrupt vaginal pH can backfire.
Effects Beyond Reproduction
The hormones produced by the ovaries, particularly estrogen, influence far more than fertility. Estrogen affects bone density, cardiovascular function, brain health, skin, the urinary tract, and mucous membranes throughout the body. This is why menopause, which brings a sharp drop in estrogen, is associated with accelerated bone loss and shifts in cardiovascular risk.
Estrogen’s role in bone health is significant enough that hormone therapy has been considered for osteoporosis prevention, though current guidelines reserve it for women at high risk who can’t take other medications. Similarly, while estrogen was once thought to protect heart health, large studies found that hormone replacement therapy did not prevent heart disease and increased the risk of certain other conditions. The takeaway is that estrogen’s reach extends to nearly every organ system, and its decline at menopause has consequences that go well beyond the end of menstrual cycles.