HCG Trigger in Fertility Treatment: Mechanism and Function
In females undergoing fertility treatment, the HCG trigger injection mimics the natural LH surge to induce final oocyte maturation and trigger ovulation approximately 36-40 hours after administration. 1, 2, 3
Primary Mechanism of Action
HCG functions virtually identically to pituitary LH, stimulating the production of gonadal steroid hormones. 3 In the context of IVF/ICSI cycles, this means:
- Final oocyte maturation occurs when HCG is administered after follicles reach approximately 18mm mean diameter, preparing eggs for retrieval 36-40 hours later 1, 2
- The corpus luteum is stimulated to produce progesterone, supporting the luteal phase and potential implantation 3
- Ovulation is triggered with a mean time to ovulation of 40.4 hours after intramuscular HCG injection 2
Specific Applications in Fertility Treatment
IVF/ICSI Cycles
- Standard protocol involves administering 5,000-10,000 IU of HCG when dominant follicles reach appropriate size (typically 18mm) to trigger final oocyte maturation before retrieval 1, 4
- Oocyte retrieval is performed 36-40 hours after trigger, timed to capture mature eggs before spontaneous ovulation 1, 2
IUI Cycles with Ovarian Stimulation
- HCG trigger is administered when the dominant follicle reaches approximately 18mm mean diameter 2
- Single IUI can be performed any time between 24-40 hours after HCG injection without compromising pregnancy rates, providing clinical flexibility 2
Hormonal Effects During Treatment
The HCG trigger creates specific hormonal changes:
- Substitutes for the mid-cycle LH surge that naturally triggers ovulation in spontaneous cycles 3
- Maintains corpus luteum function after LH secretion decreases, supporting continued progesterone production 3
- Does NOT affect fat mobilization, appetite, hunger, or body fat distribution despite historical misconceptions 3
Alternative Trigger Strategies
Recent evidence shows:
- GnRH agonist trigger can be used as an alternative in antagonist protocols, with different gene expression patterns in granulosa cells 5
- GnRH agonist trigger results in lower expression of steroidogenesis genes (CYP19A1, CYP11A1) and VEGF at oocyte retrieval compared to HCG trigger 5
- FSH co-trigger (adding FSH bolus with HCG) provides no additional benefit - a single HCG trigger is sufficient for clinical pregnancy rates 4
Administration Routes
- Both subcutaneous and intramuscular routes are effective, producing similar serum HCG levels 24 hours post-injection 6
- Subcutaneous administration (0.5mL volume) is well-tolerated in lean women (BMI <30) undergoing ovulation induction 6
Critical Distinction from Male Fertility Treatment
In males with hypogonadotropic hypogonadism, HCG serves an entirely different purpose - stimulating Leydig cells to produce testosterone while preserving fertility potential, as exogenous testosterone would suppress spermatogenesis. 7, 3 This male application is unrelated to the ovulation trigger function in females.