Intrauterine Insemination (IUI) is a common fertility treatment involving the placement of prepared sperm directly into the uterus near the time of ovulation. Success depends on factors like sperm quality and egg maturity. A follicle is a small, fluid-filled sac in the ovary that contains a developing egg. In an IUI cycle, precise timing and follicle maturity are paramount to maximize fertilization chances. Follicle size is the primary indicator that the egg inside is ready for release.
The Optimal Follicle Size for IUI Success
The size of the dominant ovarian follicle reliably predicts if the egg within it is mature and capable of fertilization. Fertility specialists have established a narrow range signaling readiness for ovulation and subsequent IUI. The generally accepted optimal diameter for a dominant follicle before triggering ovulation is between 18 and 22 millimeters (mm).
Follicles smaller than this range, typically less than 16 mm, are likely to contain an immature egg, which significantly lowers the chance of successful fertilization. Conversely, follicles exceeding 24 mm may contain an overripe or post-mature egg, which also reduces viability. The 18 to 22 mm window indicates oocyte maturation, ensuring the egg is fully developed and prepared to meet the sperm.
Not all follicles grow at the same rate, leading to the distinction between primary and dominant follicles. Primary follicles are an early stage of development, too small to be visible on ultrasound or release a mature egg. During each menstrual cycle, a cohort of these follicles is activated, but hormonal signals eventually cause only one (or a few, during a stimulated IUI cycle) to become the dominant follicle.
The dominant follicle continues to grow and swell with fluid, reaching the necessary size to rupture and release the egg. This dominant follicle is the focus of the IUI cycle, as its size dictates the timing of subsequent treatment steps. The goal is often to encourage one or two dominant follicles to reach this specific size to increase pregnancy rates while minimizing the risk of a multiple pregnancy.
Achieving and Tracking Follicle Growth
To ensure follicles reach the optimal size, IUI cycles often incorporate medications to stimulate the ovaries. These medications can be oral, such as Clomiphene Citrate or Letrozole, or injectable gonadotropins, which provide the hormones needed for follicle growth. The choice of medication and dosage is individualized, aiming to promote the development of one or two high-quality dominant follicles.
Monitoring follicle growth is done through folliculometry, using serial transvaginal ultrasounds. These scans allow the physician to measure the diameter of developing follicles and track their daily progress. Monitoring usually begins around day nine or ten of the cycle, or several days after the stimulation medication is started.
Once a follicle reaches a diameter of approximately 10 mm, it typically enters a rapid growth phase. During this time, the expected growth rate is about 1 to 2 mm per day. Tracking this growth rate is essential for determining when the dominant follicle enters the optimal 18 to 22 mm range. This careful monitoring ensures the IUI procedure is timed to coincide with the release of a fully mature egg.
Timing the Procedure: The Role of the Trigger Shot
Once ultrasound confirms the dominant follicle has reached the optimal size, the physician administers a “trigger shot” to induce final maturation and egg release. This injection typically contains human Chorionic Gonadotropin (hCG) or a GnRH agonist, which mimics the natural Luteinizing Hormone (LH) surge that occurs just before ovulation. The shot controls the timing of ovulation, moving it from a variable natural event to a predictable scheduled one.
The trigger shot initiates hormonal events that cause the mature egg to complete its final developmental stage and break free from the ovarian wall. Ovulation is highly predictable following this injection, usually occurring between 36 and 40 hours after administration. This predictable timing makes the trigger shot a necessary component of a controlled IUI cycle.
The IUI procedure is then scheduled to take place approximately 36 hours after the trigger shot. This timing places the washed and concentrated sperm into the uterus just as the mature egg is released from the follicle and enters the fallopian tube. Synchronizing sperm delivery with the brief window of egg viability maximizes the chance of successful fertilization and pregnancy.