Should You Proceed with Trigger Shot for IUI with a 15mm Lead Follicle?
No, you should not proceed with the trigger shot when your lead follicle is only 15mm—wait until you have 1-2 follicles that are clearly >15mm (ideally 16-18mm or larger) before triggering. 1
Why Follicle Size Matters
The guidelines consistently define "dominant follicles" as those >15mm, not those that are exactly 15mm. 1, 2 This distinction is critical because:
- A 15mm follicle is at the threshold, not beyond it, meaning it may not be fully mature and could yield a suboptimal oocyte or inadequate ovulation response
- The evidence-based protocols specify triggering when you have 1-2 follicles >15mm (meaning greater than, not equal to 15mm) 1, 2
- Pregnancy rates improve when you have 2 mature follicles compared to 1, with a 5% increase in pregnancy rate 1, 2
Optimal Trigger Timing Algorithm
Wait an additional 1-2 days of stimulation until:
- You have 1-2 follicles measuring 16-18mm or larger (clearly >15mm) 1, 2
- AND you have <5 follicles >10mm to minimize multiple pregnancy risk 1, 2
Then trigger with hCG and perform IUI 24-40 hours later 1, 2, 3
Critical Safety Parameters You Must Follow
Cancel the cycle entirely if you develop: 1, 2
- >2 follicles >15mm (multiple pregnancy risk jumps to 14-23%) 1, 2, 4
- >5 follicles >10mm (high-order multiple pregnancy risk becomes unacceptable) 1, 2
The data shows that with 3-4 mature follicles, multiple pregnancy risk increases to 14-23% per pregnancy, with higher-order multiples (triplets+) occurring in up to 10.6% of pregnancies. 2, 4 This dramatically increases maternal and neonatal morbidity without improving your chance of a singleton pregnancy. 2, 4
Common Pitfall to Avoid
Do not trigger prematurely just because you're eager to proceed. A single day of additional follicle growth can make the difference between a suboptimal response and optimal oocyte maturation. 5 Triggering at exactly 15mm (rather than waiting for clearly >15mm) may result in:
- Lower oocyte quality
- Failed ovulation
- Reduced pregnancy rates
- Wasted cycle
Post-Trigger Protocol
Once you have appropriate follicle development and trigger: