Follicles are small, fluid-filled sacs within a woman’s ovaries, each containing an immature egg. A “trigger shot” is a medication that induces the final maturation of eggs inside these follicles. This prepares eggs for retrieval in procedures like in vitro fertilization (IVF) or for timed ovulation in cycles such as intrauterine insemination (IUI). The precise timing of this injection is important for maximizing successful outcomes in fertility treatments.
Understanding Follicle Development and the Trigger Shot
In a natural menstrual cycle, several follicles begin to grow, but typically only one becomes dominant and releases an egg. Fertility medications, such as gonadotropins (FSH), stimulate the ovaries to encourage the growth of multiple follicles. This process, called ovarian stimulation, aims to produce several mature eggs, increasing the probability of pregnancy.
The trigger shot mimics the natural surge of luteinizing hormone (LH), signaling eggs to complete their final maturation. This injection is commonly human chorionic gonadotropin (hCG), similar in structure and function to LH. A GnRH agonist, such as leuprolide acetate (Lupron), is another type of trigger shot that stimulates the body’s own LH surge. Administering the trigger shot causes eggs to detach from the follicle wall, preparing them for retrieval or release. Ovulation or egg retrieval typically occurs about 36 hours after the trigger shot.
Determining Optimal Follicle Size for Trigger
The timing of the trigger shot is determined by follicle size, aiming to retrieve the maximum number of mature eggs. Follicles are generally ready for the trigger shot when two to three dominant follicles reach 17-20 millimeters in diameter. An optimal range of 15-22 mm for egg retrieval is also indicated.
Follicles measuring 12-19 mm on the day of the trigger shot are most likely to yield a mature egg. Eggs from follicles smaller than 16 mm are less likely to be mature, while those larger than 22 mm might be post-mature or compromised. The goal is to administer the trigger when a significant number of follicles are within this optimal range, even if not all reach the exact same size simultaneously.
Monitoring Follicle Growth
Fertility specialists track follicle development to determine trigger shot timing. Transvaginal ultrasound is the primary monitoring method, allowing visualization and measurement of follicle size and number. Scans typically begin around day 3 of the menstrual cycle to establish a baseline and continue every one to three days as follicles grow.
Blood tests measure hormone levels, particularly estradiol (estrogen). Estradiol levels rise as follicles grow and mature, providing additional information. Progesterone levels may also be monitored. Ultrasound and blood test results provide a comprehensive picture of follicular maturity and guide the decision on when to administer the trigger shot.
Consequences of Improper Trigger Timing
Administering the trigger shot at the incorrect time can significantly impact fertility treatment success. If given too early, when follicles are too small, eggs may not have completed final maturation. This can lead to retrieval of immature eggs unable to be fertilized, reducing pregnancy chances.
If administered too late, when follicles are too large, eggs may become over-mature or degenerate. There is also a risk of premature ovulation before scheduled egg retrieval, meaning eggs would be released into the fallopian tubes and become unavailable for collection. Either scenario of mistimed triggering can reduce viable eggs and embryos, potentially leading to a canceled cycle or a lower chance of pregnancy.