Does a Trigger Shot Increase the Size of the Lead Follicle?
No, a trigger shot (hCG or GnRH agonist) does not increase the size of the lead follicle after administration—it induces final oocyte maturation within follicles that have already reached adequate size, typically triggering ovulation within 36-40 hours.
Mechanism of Trigger Shot Action
The trigger shot functions to induce final oocyte maturation and ovulation, not to promote further follicular growth 1. Once administered, the cascade toward ovulation begins immediately, with the oocyte completing its final maturation steps within the existing follicle structure 1.
Optimal Follicle Size at Time of Trigger
The follicle should already be at mature size before trigger administration:
- Standard practice dictates triggering when the dominant follicle reaches approximately 18 mm mean diameter 2
- Follicles measuring 12-19 mm on the day of trigger are most likely to yield mature oocytes, whether using hCG, GnRH agonist, or kisspeptin 3
- Patients with the highest proportion of follicles in the 12-19 mm range at trigger yielded 4.7-4.9 more mature oocytes compared to those with fewer follicles in this optimal size range 3
Timing Considerations
Earlier versus later triggering based on follicle size produces equivalent outcomes:
- Triggering when the lead follicle measures 16.0-16.9 mm produces similar clinical pregnancy rates (11.9%) compared to triggering at 18.0-18.9 mm (12.1%) in IUI cycles 4
- Single IUI can be performed anytime between 24-40 hours after hCG injection without compromising pregnancy rates 2, 5
Clinical Implications
The key clinical point is that follicular growth must be completed before trigger administration. The trigger shot does not compensate for inadequate follicular development. If follicles are too small at the time of trigger, they will not suddenly grow to adequate size—instead, they may yield immature oocytes or fail to ovulate properly 3.
Common pitfall to avoid: Administering trigger too early when follicles are <12 mm significantly reduces the likelihood of retrieving mature oocytes, as these smaller follicles contribute minimally to mature oocyte yield 3.