Ovarian follicles are small, fluid-filled sacs within the ovaries, each containing an immature egg, or oocyte. In a natural menstrual cycle, only one follicle typically reaches full maturity. During an In Vitro Fertilization (IVF) cycle, hormone medications are administered to encourage the development of many follicles simultaneously. Measuring the size of these follicles helps physicians determine the exact moment the eggs are ready for retrieval. The ultimate goal is to collect the maximum number of mature eggs, and this timing depends entirely on follicle growth.
The Target Size for Egg Maturity
A successful IVF egg retrieval depends on achieving an optimal size range for the developing follicles. The generally accepted range for containing a mature egg is approximately 15 millimeters (mm) to 22 mm in diameter. Follicles within this window have had sufficient time and hormonal exposure to finalize the oocyte maturation process.
The decision to proceed is typically made when the “lead” follicles—the largest ones—reach about 17 mm to 18 mm. Physicians look for at least two to three follicles in this range before administering the final maturation injection. This target size ensures the largest follicles are ready while allowing slightly smaller ones to catch up during the final hours of maturation.
Follicles often grow at different rates, leading to a heterogeneous group of sizes at retrieval. While 15-22 mm is the ideal target, follicles as small as 14 mm often yield mature eggs and are still collected. The fertility team aims for a cohort of follicles that are collectively ready, not just one specific measurement.
Monitoring Follicle Growth During Stimulation
Follicle growth is tracked meticulously throughout the ovarian stimulation phase using transvaginal ultrasound. This imaging technique provides clear, precise measurements of the fluid-filled sacs. Monitoring typically begins a few days after starting stimulation medications and continues with increasing frequency, often requiring daily or every-other-day clinic visits toward the end of the phase.
During each visit, the physician measures the diameter of all follicles larger than a few millimeters. Follicles generally grow at a rate of about 1.7 mm per day once they reach 10 mm or more. These measurements, combined with blood tests for hormone levels like estradiol, allow the medical team to assess the patient’s response to the medication protocol.
If the follicles are growing too slowly or too quickly, the medication dosage of Follicle-Stimulating Hormone (FSH) or Luteinizing Hormone (LH) is adjusted accordingly. This fine-tuning promotes steady, synchronous growth across the entire cohort of developing follicles. The goal of this intensive monitoring is to determine the perfect moment for the final intervention before the eggs are naturally released.
Why Size Correlates with Egg Quality
The diameter of the follicle serves as a reliable marker for oocyte maturity because the follicle provides the necessary microenvironment for maturation. As the follicle increases in size, the follicular fluid volume expands, and the surrounding granulosa cells proliferate. These cells produce the hormones and growth factors required for the egg to complete its development.
Follicles that are too small, typically less than 14 mm, are significantly less likely to contain a fully mature egg ready for fertilization. An immature egg, or germinal vesicle (GV) stage oocyte, cannot be successfully fertilized. Conversely, follicles exceeding 22 mm may indicate an egg that has become post-mature or atretic, meaning it may have begun to degenerate.
While the largest follicles have the highest probability of yielding a mature egg, a smaller follicle that is successfully fertilized can still lead to a healthy embryo. Oocytes retrieved from smaller follicles, if mature, have comparable developmental potential to those from larger follicles. However, the proportion of mature eggs retrieved drops significantly as follicle size decreases, highlighting why the size threshold is closely observed.
Next Steps After Reaching Optimal Size
Once monitoring confirms the largest follicles have reached the 17-18 mm range, the final step is to administer the “trigger shot.” This injection contains a hormone, most commonly human chorionic gonadotropin (hCG) or a GnRH agonist, designed to mimic the natural Luteinizing Hormone (LH) surge that precedes ovulation. The trigger shot initiates the final maturation of the eggs.
The timing of this injection is critical, as it starts a precise biological clock for egg development. The retrieval procedure must occur approximately 34 to 36 hours after the trigger shot is administered. This tight window ensures that the eggs have completed their final maturation process but are collected just before the ovaries would naturally release them.
Missing this specific timing window by even a few hours can result in the retrieval of immature eggs or the spontaneous release of the eggs into the abdominal cavity, which would necessitate the cancellation of the cycle. The trigger shot coordinates the biological process with the clinical procedure.