Ovarian follicles are small, fluid-filled sacs within the ovaries, each containing an immature egg cell. These structures support egg maturation and produce hormones like estrogen, regulating the menstrual cycle. Monitoring the size and number of these follicles using transvaginal ultrasound is a standard procedure in reproductive medicine. This imaging technique allows professionals to track the development of these sacs for managing fertility treatments and assessing reproductive health.
The Purpose of Follicle Tracking
Follicle tracking, or follicular monitoring, links the physical growth of the follicle to its biological function. In a natural cycle, monitoring helps predict the precise timing of ovulation, as the largest follicle, known as the dominant follicle, is the one most likely to release an egg. For those undergoing fertility treatments, tracking ensures the ovaries are responding appropriately to stimulation medications.
The measurements guide the dosage of hormonal drugs and help prevent complications like ovarian hyperstimulation syndrome (OHSS). For procedures like In Vitro Fertilization (IVF) or Intrauterine Insemination (IUI), tracking pinpoints the optimal moment for egg retrieval or insemination. Tracking also identifies potential abnormalities, such as large, unruptured follicles that may become ovarian cysts.
Understanding Ultrasound Measurements and Terminology
To read follicle size on an ultrasound screen, one must first recognize the units and methods used for measurement. Follicle dimensions are measured in millimeters (mm). The follicle appears as a dark, spheroid structure due to its fluid content, and the technician uses electronic calipers to measure its diameter.
Because follicles are three-dimensional, slightly irregular structures, measurement involves taking at least two perpendicular dimensions—length and width—to calculate an average diameter. A third depth measurement is sometimes taken, and the average of all three dimensions represents the follicle’s size. During stimulated cycles, multiple follicles are individually measured and recorded, often labeled with abbreviations like “D” for dominant or tracked by their coordinates on the ovary.
Interpreting Follicle Size Thresholds
The size of a follicle on the ultrasound image directly correlates with the stage of egg maturation and its readiness for release. Follicles less than 10 mm in diameter are considered to be in the early growth phase. At the start of a cycle, a baseline scan confirms that no follicles are prematurely large and that multiple small antral follicles, often between 3 mm and 8 mm, are present.
As the cycle progresses, a single follicle is selected to become dominant, growing more rapidly than the others. Follicles in the 10 mm to 17 mm range represent the active, growing phase, increasing in size by an average of 1 mm to 2 mm per day. This rapid growth phase is carefully monitored to determine the timing for the final maturation step.
The maturity threshold indicates that the egg inside is ready for fertilization. For natural ovulation or IUI, the dominant follicle is generally considered mature when it reaches a diameter between 18 mm and 25 mm. In an IVF cycle, the target size for egg retrieval is often slightly different, with follicles between 16 mm and 22 mm typically yielding mature eggs. Follicles that grow beyond 25 mm may indicate a hyper-response or be considered functional cysts that have failed to rupture.
Next Steps Based on Follicle Size
Once the lead follicles reach the pre-determined size threshold, commonly 18 mm to 22 mm, the next clinical action is scheduled. This size signals that the egg within the follicle has undergone final maturation and is ready to be released. The most common step is the administration of a “trigger shot,” a hormone injection (often human chorionic gonadotropin, or hCG), which mimics the body’s natural luteinizing hormone surge.
The trigger shot is precisely timed to induce final maturation and ovulation or egg release approximately 36 to 40 hours later. For IUI, this timing directs the insemination procedure, while for IVF, it sets the schedule for the egg retrieval surgery. If follicles are growing too slowly or remain too small, the medication regimen may be adjusted, and monitoring continues until the desired size is achieved. Conversely, if a patient has an excessive number of large follicles, the cycle may be modified or canceled to mitigate the risk of complications like OHSS.