The trigger shot for IUI is typically given when the lead follicle reaches 18 to 22 mm in diameter, though recent research suggests waiting until it measures closer to 21 to 22 mm may significantly improve your chances of pregnancy. The insemination itself is then scheduled roughly 36 to 42 hours later, timed to coincide with ovulation. Getting this sequence right is one of the few variables in IUI that you and your clinic can directly control.
Follicle Size: The Primary Trigger
Your clinic monitors follicle growth with transvaginal ultrasound, usually every few days during a stimulated cycle. Most clinics consider a follicle “mature” once it hits 18 mm, but triggering at that size may not be optimal. An analysis of 1,676 clomiphene-IUI cycles found that the odds of clinical pregnancy were 2.3 times higher when the lead follicle measured 21 to 22 mm at the time of the trigger, compared to 19 to 20 mm. Follicles larger than 22 mm showed a similar benefit, with 2.2 times higher odds.
That same study identified 22.1 mm as the threshold with the best balance of sensitivity and specificity for predicting pregnancy. In practical terms, this means your clinic may ask you to come back for one more monitoring appointment rather than triggering as soon as the follicle crosses 18 mm. The difference of a couple of millimeters can matter.
If you’re using letrozole or injectable hormones instead of clomiphene, the general follicle size targets are similar, though your doctor may adjust based on how many follicles are developing. When more than three or four follicles exceed 18 mm, the cycle is often canceled to reduce the risk of a high-order multiple pregnancy.
Endometrial Thickness at Trigger Time
While follicle size drives the trigger decision, your uterine lining is checked at the same ultrasound appointments. Cycles that resulted in a live birth had an average lining thickness of 9.5 mm on the day of the trigger, compared to 8.9 mm in cycles that did not. The highest live birth rates were seen with linings in the 13 to 15 mm range.
That said, pregnancies occurred at every thickness, including linings of 5 mm or less. A large study from Fertility and Sterility concluded that cycles should not be canceled solely because of a thin lining. So while a thicker endometrium is encouraging, a thinner one does not mean the cycle is futile.
How Many Hours Between Trigger and IUI
The trigger shot causes ovulation approximately 36 to 40 hours after injection. Clinics traditionally schedule the insemination at the 36-hour mark, but the evidence on exact timing is more flexible than many patients realize.
A randomized trial comparing insemination at 36 hours versus 42 hours found significantly higher pregnancy rates in the 42-hour group: a 32% chemical pregnancy rate compared to 14%. Another study comparing 36 and 48 hours also found better outcomes with the later timing. However, a large systematic review of over 2,200 women across 18 trials concluded there was no significant difference in outcomes anywhere in the 24- to 48-hour window.
The takeaway: the window is wider than it feels. If your clinic schedules you at 34 hours or 40 hours, both fall well within the effective range. You do not need to panic if your appointment shifts by a few hours.
Single vs. Double Insemination
Some clinics offer two inseminations per cycle, typically one at 18 to 24 hours post-trigger and a second at 36 to 48 hours. A Cochrane review of nine trials and over 2,700 participants found that double IUI may modestly increase clinical pregnancy rates per cycle (about 1.5 times higher odds), but the evidence was low quality. For live birth rates specifically, there was no clear benefit to the second insemination.
Double IUI also means two visits, two semen collections, and in some cases higher costs. Most clinics default to a single, well-timed insemination unless there’s a specific reason to try two.
Trigger Shot vs. Natural LH Surge
Not every IUI cycle uses a trigger shot. Some clinics monitor for your body’s natural LH surge using blood work or urine ovulation kits, then schedule the insemination accordingly. A retrospective analysis of over 6,200 letrozole-based IUI cycles compared three groups: patients who received only a trigger shot, patients who had a natural LH surge without a trigger, and patients who had both. Clinical pregnancy rates were virtually identical across all three groups, ranging from 15.6% to 16.7%.
The practical advantage of a trigger shot is predictability. It lets your clinic schedule the insemination precisely rather than waiting for a surge that might happen overnight or on a weekend. But if your body surges on its own before the planned trigger, that’s not a problem. The data suggests outcomes are the same either way.
When Premature Ovulation Disrupts the Plan
About 24% of stimulated IUI cycles are complicated by a premature LH surge, where the body starts the ovulation process before the follicle has reached its target size. This can lead to cycle cancellation or poorly timed insemination.
To prevent this, some clinics add a medication that temporarily blocks the brain’s signal to ovulate. These drugs can be started on a fixed day (around cycle day 6) or introduced flexibly once follicles reach a certain size, and they suppress LH within hours. This keeps the ovulation timeline under your clinic’s control until the follicle is ready. If a premature surge does begin, triggering with the shot after the surge starts (rather than canceling the cycle) can still salvage the attempt, since the natural surge can take up to two days to complete ovulation.
What the Trigger Shot Feels Like
The injection itself is subcutaneous (into belly fat) or intramuscular (into the upper buttock or thigh), depending on the formulation your clinic prescribes. The most common side effects are bloating, mild pelvic discomfort, and soreness at the injection site. These typically start within a day and resolve on their own. Because the trigger shot contains the same hormone your body produces in early pregnancy, it can also cause a false positive on a home pregnancy test for up to 10 to 14 days after injection.