The process of assisted reproductive technology (ART), such as In Vitro Fertilization (IVF) or Intrauterine Insemination (IUI), relies on precise hormonal control to maximize success. Follicles are small, fluid-filled sacs within the ovaries that house and nurture developing egg cells (oocytes). The “trigger shot,” typically containing Human Chorionic Gonadotropin (HCG), is a hormone injection administered to induce the final maturation of these eggs. Timing this injection is crucial to ensure the eggs are ready for retrieval or fertilization, making follicle size a determining factor in the cycle’s progression.
The Role of Follicles in Ovulation Induction
Follicles are the functional units of the ovary. Their development, known as folliculogenesis, involves a sequence of growth stages. While a natural cycle typically yields one dominant follicle and a single egg, fertility treatments use stimulating hormones like Follicle-Stimulating Hormone (FSH) to encourage multiple follicles to grow simultaneously.
This controlled ovarian stimulation aims to develop a cohort of follicles, increasing the number of available oocytes. Retrieving or releasing multiple mature eggs improves the probability of a successful pregnancy. The trigger shot timing ensures the enclosed egg has achieved necessary biological maturity. If a follicle is too small, the egg may be immature; if too large, the egg may be post-mature and less viable.
Target Follicle Sizes for the HCG Trigger
The precise size required for a follicle ready for the trigger shot varies depending on the specific fertility treatment.
IUI Cycles
For cycles involving IUI, the goal is to mature a smaller number of eggs. The trigger is typically administered when at least one follicle reaches a diameter of 18 millimeters (mm) or more. While some clinics may aim for sizes up to 24 mm, 18 mm is often the minimum threshold to promote the release of a healthy, fertilizable egg.
IVF Cycles
In an IVF cycle, the objective is to collect as many mature eggs as possible. The trigger is commonly given when two to three of the leading follicles have reached 17 mm to 18 mm. Follicles measuring between 12 mm and 19 mm on the day of the trigger are the most likely to yield a mature egg upon retrieval. This range is chosen because smaller follicles (e.g., 12–16 mm) will continue to grow and mature in the approximately 36 hours between the injection and the egg retrieval procedure. Maximizing the number of follicles within this 12–19 mm window optimizes the potential yield. Follicles much larger than 20 mm may contain eggs past their optimal maturity and less likely to fertilize successfully.
Monitoring and Assessment Methods
Determining the precise moment to administer the trigger shot requires close monitoring of the developing follicles. The primary method used by fertility specialists is transvaginal ultrasound, which provides images of the ovaries. During the stimulation phase, patients undergo frequent scans to measure the diameter of each active follicle.
Follicle measurements are conducted by averaging two perpendicular dimensions to ensure accuracy, and the growth rate is tracked to anticipate the target size. The ultrasound also allows the specialist to assess the uterine lining (endometrium), which should thicken in response to rising hormone levels.
Complementary to ultrasound, blood tests measure the serum Estradiol (E2) level, a form of estrogen produced by the cells within the follicles. Rising E2 levels indicate that the follicles are healthy and actively maturing the eggs. Clinically, each mature follicle typically produces 200 to 300 picograms per milliliter (pg/mL) of E2. The total E2 level on the day of the trigger, often peaking between 1,000 and 2,500 pg/mL, serves as a biochemical confirmation that sufficient eggs are ready for final maturation.
The HCG Trigger Shot and Subsequent Timing
Once ultrasound and E2 levels confirm that follicles have reached the desired size, the HCG trigger shot is administered. HCG structurally mimics Luteinizing Hormone (LH), the hormone that naturally surges mid-cycle to prompt ovulation. This injected surge causes the oocytes within the mature follicles to complete their final cell division (meiosis), preparing them for fertilization.
The timing of this injection is critical because eggs ovulate approximately 36 to 40 hours afterward. In IVF, the egg retrieval procedure is strictly scheduled for 34 to 36 hours after the injection to ensure collection before natural ovulation. In IUI cycles, insemination is timed within this same window, allowing sperm to be present when the mature egg is released.