IUI, or intrauterine insemination, is a fertility treatment where specially prepared sperm is placed directly into the uterus around the time of ovulation. It bypasses several natural barriers that can prevent sperm from reaching the egg, giving conception a better chance. Per-cycle success rates range from about 9% to 20% depending on age, making it less effective than IVF but far less invasive and expensive.
How IUI Works
In natural conception, sperm travel from the vagina through the cervix, into the uterus, and up to the fallopian tubes where fertilization happens. Along the way, most sperm don’t survive. IUI shortens that journey considerably by depositing a concentrated sample of the healthiest sperm directly into the uterus, close to the fallopian tubes.
Before insemination, the sperm sample goes through a lab process called “washing.” This separates the most active, healthy sperm from the rest of the semen fluid. Labs typically use one of three methods: a swim-up technique where motile sperm naturally swim into a clean layer of fluid, a density gradient that sorts sperm by spinning them through layers of solution, or a simple wash-and-spin method. The result is a small, concentrated dose of high-quality sperm ready for insemination.
The actual insemination takes just a few minutes. You lie on an exam table, a speculum is inserted (similar to a Pap smear), and a thin, flexible catheter is threaded through the cervix into the uterus. The washed sperm is then injected. You may be asked to lie still for 10 to 30 minutes afterward, and that’s it. From there, fertilization and implantation happen naturally, just as they would in unassisted conception.
Who Is a Good Candidate
IUI is often the first step for couples or individuals who haven’t conceived after at least a year of trying. It works best for specific situations where sperm just need a head start reaching the egg, rather than cases where more complex barriers exist.
The most common reasons people try IUI include:
- Unexplained infertility: when all standard tests come back normal but pregnancy hasn’t happened
- Mild male factor infertility: lower-than-average sperm count, motility, or shape, though the sample still has at least 5 million sperm per milliliter
- Cervical issues: thick cervical mucus or scarring that blocks sperm from passing through
- Ovulation problems: conditions like polycystic ovary syndrome (PCOS) where eggs aren’t released regularly
- Mild to minimal endometriosis: early-stage tissue growth outside the uterus that interferes with fertility
- Use of donor sperm: for single parents by choice, same-sex couples, or couples using a sperm donor
IUI is generally not recommended when both fallopian tubes are blocked, endometriosis is moderate to severe, or the sperm count after washing falls below about 1 million per milliliter. In those cases, IVF is typically the better option.
The Full Cycle Timeline
An IUI cycle takes roughly two to four weeks from start to finish. Some cycles use your natural ovulation, but most involve medication to boost the number of eggs released and control the timing precisely.
If medication is used, you’ll typically start oral pills early in your cycle. The two most common options work slightly differently. One (clomiphene) tricks the brain into releasing more of the hormones that stimulate egg growth by blocking estrogen signals. The other (letrozole) reduces estrogen production directly, prompting the same hormonal response. Letrozole has gained popularity because it doesn’t thin the uterine lining or reduce cervical mucus the way clomiphene can.
Around day 8 of your cycle, ultrasound monitoring begins to track how your follicles (the fluid-filled sacs containing eggs) are growing. You’ll have scans every other day, then daily once a follicle reaches about 14 mm. When at least one follicle hits 17 to 18 mm, a trigger shot of the hormone hCG is given to prompt ovulation within 36 to 48 hours. The insemination is scheduled to coincide with that window, typically 6 to 12 hours after ovulation signs are detected. After the procedure, you may take progesterone supplements to support the uterine lining during the two-week wait before a pregnancy test.
What It Feels Like
Most people compare the procedure to a routine Pap smear. You may feel mild cramping as the catheter passes through the cervix, and some experience light spotting for a day or two afterward. There’s no anesthesia involved and no recovery period. Most people return to normal activities immediately, though you might want to take it easy if cramping bothers you.
Success Rates by Age
Age is the single biggest factor in IUI success. An analysis of over 4,200 insemination cycles found the following pregnancy rates per cycle:
- Under 25: about 20% per cycle
- 25 to 29: about 13% per cycle
- 30 to 34: about 11% per cycle
- 35 to 39: about 9% per cycle
- 40 to 41: about 9% per cycle
- 42 to 43: about 6% per cycle
- Over 43: about 3.5% per cycle
These are per-cycle numbers, so cumulative odds improve over multiple attempts. Most fertility specialists suggest trying three to six IUI cycles before considering IVF. By comparison, IVF achieves a per-cycle live birth rate roughly two to three times higher, around 32% versus about 15% for IUI in one comparative study of unexplained infertility. But IUI costs a fraction of what IVF does, which is why it’s often tried first.
Multiple Pregnancy Risk
The most significant risk with IUI comes not from the insemination itself but from the fertility medications that stimulate multiple eggs. More mature follicles at the time of insemination means a higher chance of twins or triplets, and this is where things require careful monitoring.
For women under 38, going from one mature follicle to five at the time of IUI increased the pregnancy rate modestly, from about 15% to 22%. But the multiple pregnancy rate jumped dramatically, from 0.6% to 6.5% per cycle. Among those who did get pregnant with five follicles, nearly a quarter carried twins and about 11% carried triplets or more. With more than three follicles present, over one in four pregnancies in this age group were multiples.
For women over 40, the math shifts. Up to four follicles tripled the odds of pregnancy while keeping the multiple gestation risk under 12% per pregnancy. This is why your doctor will monitor follicle development closely and may cancel a cycle if too many eggs are maturing, particularly if you’re younger.
How IUI Compares to IVF
IUI and IVF serve different situations, and one isn’t universally better than the other. IUI is less invasive (no egg retrieval, no sedation, no embryo transfer), takes less time per cycle, and costs significantly less. It’s a reasonable first step when fertility barriers are mild or moderate.
IVF produces higher pregnancy rates per cycle. In a study comparing the two approaches for unexplained infertility, IVF achieved a 39% live birth rate per cycle compared to about 15% for IUI. However, many guidelines suggest starting with IUI precisely because it’s simpler and cheaper. The trade-off is that about 80% of people who try IUI without success will eventually move to IVF anyway, adding the IUI costs on top of their total treatment expense.
For people with blocked fallopian tubes, severe male factor infertility, or advanced reproductive age, skipping straight to IVF often makes more sense. For younger patients with unexplained infertility, mild ovulation issues, or cervical factors, a few rounds of IUI can be a practical and lower-stress place to start.