A dominant follicle is a fluid-filled sac within the ovary containing an immature egg, central to the menstrual cycle. Each cycle, multiple ovarian follicles begin to grow, but typically only one, the dominant follicle, fully matures to house and prepare the egg for release during ovulation. Understanding its function provides insights into reproductive health.
The Journey of Follicle Growth
Follicle development begins with small, immature follicles within the ovaries. These primordial follicles are present from birth and progress through stages: primary, secondary, and antral. At the start of each menstrual cycle, several antral follicles are recruited and begin to grow under the influence of follicle-stimulating hormone (FSH). As these follicles grow, they compete for resources and hormonal signals. By the mid-follicular phase, one follicle grows larger and more rapidly than the others, signifying its “dominance.” It becomes more sensitive to FSH and produces increasing estrogen, which suppresses the growth of other follicles, causing them to cease development.
The Dominant Follicle’s Role in Ovulation
Once selected, the dominant follicle matures, reaching 18 to 25 millimeters before ovulation. As it grows, it produces significant estrogen, signaling the brain to trigger a luteinizing hormone (LH) surge, which directly triggers ovulation. The LH surge causes the follicle wall to weaken and rupture, releasing the mature egg from the ovary. This marks ovulation, making the egg ready for potential fertilization. After egg release, the ruptured follicle’s remaining cells form the corpus luteum, a temporary structure producing progesterone to support potential pregnancy.
Identifying and Tracking the Dominant Follicle
Clinicians primarily identify and monitor the dominant follicle through transvaginal ultrasound imaging, which allows direct visualization of the ovaries and growing follicles. During an ultrasound, specialists assess the size, shape, and fluid content of the follicles to identify the one most likely to ovulate. A mature dominant follicle measures 18 to 25 millimeters in diameter just before ovulation. In addition to imaging, hormone level monitoring, particularly estrogen, provides supplementary information. As it grows, it produces increasing estrogen, so rising levels indicate its progressive maturation.
Why the Dominant Follicle Matters for Fertility
The presence and healthy development of a dominant follicle are fundamental for natural conception. Without a properly matured dominant follicle, ovulation cannot occur, and no egg is available for fertilization. Its timely growth and rupture are essential for a woman to become pregnant. For individuals undergoing assisted reproductive technologies like in vitro fertilization (IVF), understanding the dominant follicle is important. While IVF protocols often aim for multiple mature follicles, the natural tendency for one follicle to dominate still plays a role in ovarian stimulation responses. Monitoring the dominant follicle helps in precisely timing intercourse or fertility treatments, optimizing the chances of successful conception.