IUI, or intrauterine insemination, is typically performed about 24 to 36 hours after your body signals ovulation is approaching. But “when to do IUI” has two layers: when during your menstrual cycle the procedure is timed, and when in your fertility journey it makes sense to try. Both answers matter, and the timing details can directly affect your chances of success.
Who Should Consider IUI
IUI is considered a first-line fertility treatment because it’s simpler, less invasive, and far less expensive than IVF. It works by placing washed, concentrated sperm directly into the uterus, bypassing the cervix and shortening the distance sperm need to travel to reach the egg.
The most common reasons doctors recommend IUI include cervical factor infertility (where cervical mucus or anatomy makes it harder for sperm to enter the uterus), mild to moderate male factor infertility such as low sperm count or reduced motility, ovulation problems, minimal to mild endometriosis, and unexplained subfertility, where standard testing hasn’t identified a clear cause. For same-sex couples and single parents using donor sperm, IUI is often the starting point before considering more intensive options.
IUI requires at least one open fallopian tube to work. Before starting, your doctor will typically order an HSG, a low-dose X-ray where contrast dye is pushed through the uterus and tubes. This confirms the tubes are open and the uterine cavity looks normal. If both tubes are blocked, IUI won’t help and IVF becomes the better path.
Timing During Your Cycle
The goal is to get sperm into the uterus as close to ovulation as possible. There are two main ways clinics pinpoint that window: tracking your natural LH surge or using a trigger shot to force ovulation on a predictable schedule.
Your body releases a surge of luteinizing hormone (LH) roughly 24 to 36 hours before the egg is released. Most clinics schedule the insemination for one day after the LH surge is detected, either through blood work or an at-home ovulation predictor kit. The catch is that the actual time between the LH surge and follicle rupture varies considerably from person to person, ranging anywhere from 22 to 56 hours with an average around 34 hours. This variability is why monitoring matters so much.
When medications are used to stimulate ovulation, a trigger shot (injectable hCG) is given once ultrasound shows a follicle has reached the right size. The standard window is to perform the IUI 36 to 40 hours after the injection. This approach gives clinics more control over timing since ovulation becomes more predictable.
What Your Doctor Looks for on Ultrasound
Before giving the green light for insemination or a trigger shot, your clinic will measure your follicles via transvaginal ultrasound. The target size is 18 to 22 millimeters in diameter. Follicles in this range are associated with significantly higher pregnancy rates compared to those that are larger or smaller. In one study, 76.8% of positive pregnancy markers came from cycles where follicles measured 18 to 22 mm. This monitoring typically involves two to three ultrasound visits in the days leading up to ovulation.
What Happens on Procedure Day
On the day of insemination, the male partner (or donor) provides a semen sample, ideally after two to three days of abstinence. While older WHO guidelines recommend two to seven days, research on IUI outcomes specifically suggests that fewer than three days of abstinence is linked to the highest pregnancy rates. Longer abstinence doesn’t necessarily mean a better sample. It can actually reduce sperm motility.
Once collected, the sample needs to reach the lab quickly. WHO guidelines set a 60-minute limit between collection and lab processing. The sample is then “washed,” a process that separates the healthiest, most motile sperm from the rest of the semen fluid. Research shows that completing the insemination within 90 minutes of the original collection is associated with better outcomes. Some data suggests that keeping the post-wash incubation period under 60 minutes before the actual insemination is ideal.
The procedure itself takes only a few minutes. A thin catheter is passed through the cervix, and the concentrated sperm sample is deposited directly into the uterus. Most people describe it as feeling similar to a Pap smear, with mild cramping that resolves quickly.
Success Rates by Age
IUI success rates are modest per cycle but add up over multiple attempts. For women under 25, first-cycle pregnancy rates are around 25%. That number drops to about 12% to 15% for women aged 25 to 29, and roughly 10% to 12% for women 30 to 34. By ages 35 to 39, per-cycle rates sit near 9%, and they remain surprisingly similar for women aged 40 and 41. The sharpest decline happens after 42, where rates fall to about 4% per cycle, and past 43, success with IUI is extremely rare.
One encouraging finding: for women up to age 41, cumulative success rates across multiple cycles remained stable. After three cycles, women aged 40 to 41 had outcomes comparable to those of women in their late 30s. This is part of why doctors often recommend trying several rounds before moving on.
How Many Cycles Before Moving to IVF
Most fertility specialists recommend three to six IUI cycles before transitioning to IVF, but age is the biggest factor in that decision. If you’re under 35 with no major diagnosis, three to four cycles is a common recommendation. At 35 to 37, many doctors suggest reassessing after two to three unsuccessful attempts, particularly if time feels pressing. At 38 and older, some specialists recommend going directly to IVF or trying just one to two IUI cycles before switching, because egg quality declines faster and each month carries more weight.
The logic is straightforward: if IUI is going to work for you, it will most likely work within the first few cycles. Continuing beyond six cycles rarely changes the outcome and delays access to treatments with higher per-cycle success rates.
The Two-Week Wait After IUI
After the procedure, you’ll enter what fertility communities call the “two-week wait.” Even if implantation happens within a week, the pregnancy hormone (hCG) needs time to build to detectable levels. Testing too early often produces false negatives, and if you received a trigger shot, residual hCG from the injection can cause a false positive.
Wait at least 14 days after your IUI before taking a home pregnancy test. Most clinics will also schedule a blood test at that 14-day mark, which can detect lower levels of hCG than a urine test and confirm results more reliably. It’s a long two weeks, but testing earlier creates more confusion than clarity.