Intrauterine Insemination (IUI) is a fertility treatment that places sperm directly into the uterus to facilitate fertilization. This procedure aims to increase the number of sperm reaching the fallopian tubes, improving the chances of an egg being fertilized. Successful IUI relies on precise timing and optimal conditions, including ovarian follicle development.
Ideal Follicle Size for Trigger
The size of ovarian follicles is a factor in IUI success. Follicles are small sacs within the ovaries that contain immature eggs. For a trigger shot, the ideal follicle size typically ranges from 18 to 22 millimeters (mm) in diameter. Some sources indicate a slightly broader range of 18 to 24 mm. This size range suggests the egg inside the follicle has reached a mature stage, making it ready for ovulation and increasing the likelihood of successful fertilization.
When follicles reach this optimal size, they are considered “dominant” and contain a mature egg capable of being fertilized. The goal in an IUI cycle is to stimulate the development of one or a few such mature follicles. While a larger follicle might seem beneficial, follicles over 25 mm could indicate over-maturation, potentially reducing egg quality. Conversely, follicles under 18 mm may contain immature eggs, which are less likely to lead to a successful pregnancy.
Monitoring Follicle Development
Fertility specialists monitor follicle growth to determine the optimal time for administering the trigger shot and performing IUI. This monitoring primarily involves transvaginal ultrasounds, which visualize and measure the follicles. Ultrasounds track both the size and number of developing follicles.
Monitoring typically begins around day 6-8 of the menstrual cycle or after fertility medication starts. Measurements are usually taken every one to three days to track the growth rate, often around 1-2 mm per day. Alongside ultrasounds, blood tests sometimes monitor hormone levels, such as estrogen, which rises as follicles develop and provides additional maturity information.
The Trigger Shot and Its Purpose
The trigger shot is an injection of human chorionic gonadotropin (hCG). This hormone mimics the natural luteinizing hormone (LH) surge, inducing the final maturation and release of eggs from ovarian follicles. The trigger shot ensures eggs are mature and ready for ovulation in IUI or retrieval in IVF procedures.
Precise timing of the trigger shot is important for IUI success. Once dominant follicles reach the desired size, the trigger shot is administered. Ovulation typically occurs 36 to 40 hours after the injection. The IUI procedure is then scheduled to coincide with this expected ovulation, usually between 24 and 36 hours after the trigger shot.
When Follicle Size Isn’t Optimal
Sometimes, follicles do not reach the desired size or present other challenges. If follicles remain too small, they may contain immature eggs, potentially leading to cycle cancellation or continued medication. A follicle size under 18 mm at the time of trigger significantly reduces the chances of a successful pregnancy.
Conversely, if too many follicles develop or grow too large, there is an increased risk of multiple pregnancies, such as twins or higher-order multiples. Multiple pregnancies carry increased risks for the pregnant individual and babies. In situations with an excessive number of mature follicles, particularly more than three in women under 40, a fertility specialist may recommend canceling the IUI cycle to avoid these risks. This prioritizes patient safety and the likelihood of a healthy singleton pregnancy.