The trigger shot, which contains human chorionic gonadotropin (hCG), is a common medication used in fertility treatments such as Intrauterine Insemination (IUI) and In Vitro Fertilization (IVF). Its primary function is to mimic the body’s natural luteinizing hormone (LH) surge, which is the signal that prompts the final maturation of an egg. By administering this injection, fertility specialists can precisely control the timing of the egg’s readiness. The success of the treatment cycle relies heavily on knowing whether this shot induced the release of a mature egg.
The Critical Timeline for Ovulation
The timing of the trigger shot’s effect is remarkably consistent and allows for the precise scheduling of fertility procedures. Ovulation, the release of the egg from the ovarian follicle, is expected to occur approximately 36 to 40 hours after the hCG injection is administered. For patients undergoing IUI or timed intercourse, the procedure or contact is scheduled to coincide with this specific 36-to-40-hour period to maximize the chance of the sperm meeting the newly released egg. In an IVF cycle, egg retrieval is typically scheduled around 34 to 36 hours after the injection, ensuring the eggs are mature but collected just before they are spontaneously released.
Physical Indicators of Trigger Shot Success
Some patients look for physical changes that suggest the medication is working. A mild, temporary discomfort or cramping in the lower abdomen, often called mittelschmerz, may be felt as the follicle ruptures and releases the egg. Changes in cervical mucus, such as an increase in volume or a shift to a clear, slippery, egg-white consistency, can also be a sign of the hormonal cascade leading to ovulation. A slight elevation in basal body temperature (BBT) may be observed after the expected ovulation time, reflecting the rise in progesterone. However, these physical indicators are subjective and vary widely among individuals, meaning they are not considered definitive proof of success.
Objective Confirmation Methods
Fertility clinics use objective methods to definitively confirm that ovulation has occurred after the trigger shot. The most common method involves a blood test to measure the level of progesterone (P4). This test is typically performed seven days after the expected ovulation date, or about eight to nine days post-trigger. A significant rise in progesterone confirms the formation of the corpus luteum, the structure that remains in the ovary after the egg is released and produces P4.
Transvaginal Ultrasound
A transvaginal ultrasound is also used for confirmation. During the scan, the physician looks for visual evidence that the dominant follicle, measured before the shot, has collapsed. The presence of free fluid in the cul-de-sac, the space behind the uterus, is also a sign that the follicle has ruptured and released its contents.
Understanding False Positives and Residual hCG
The most common source of confusion and anxiety for patients is the potential for a false positive result on a home pregnancy test (HPT) taken too early. The trigger shot contains human chorionic gonadotropin (hCG), which is the exact hormone detected by all pregnancy tests. Therefore, if a patient uses an HPT in the days immediately following the injection, the test will likely register a positive result due to the residual medication, not an actual pregnancy.
The hCG from the shot must be fully metabolized and cleared from the body before an HPT can accurately reflect a new pregnancy. The time it takes for the exogenous hCG to leave the system depends on the dosage given, but it can take anywhere from 10 to 14 days. This period is why clinics strongly recommend waiting for the official blood test, often scheduled around 14 days post-trigger, to get a definitive result. Some individuals choose to “test out the trigger” by taking daily HPTs to watch the positive line fade away, hoping to see it return and darken, which would indicate new, pregnancy-related hCG production.