IUI Explained: How Intrauterine Insemination Actually Works

The honest picture, with numbers

Intrauterine insemination is the simplest form of assisted conception. Sperm — from a partner or a donor — is prepared in the laboratory so that the better-quality sperm are separated from those that are sluggish, non-motile or abnormally shaped, and the prepared sample is placed directly into the uterus around the time of ovulation. The HFEA summarises the trade-off in one line: success rates for IUI are generally around a third of those for IVF, and one cycle of IUI typically costs about a quarter of one IVF cycle.

What the evidence actually supports depends heavily on which IUI you mean. The 2020 Cochrane review covering 15 trials and 2,068 women found:

• IUI in a natural cycle versus expectant management in a natural cycle: uncertain. Odds ratio 1.60 (95% CI 0.92 to 2.78), one trial, 334 women, low-quality evidence. If the chance of live birth with expectant management is 16%, IUI would give somewhere between 15% and 34%.
• IUI in a stimulated cycle versus timed intercourse in a stimulated cycle: uncertain. OR 1.59 (95% CI 0.88 to 2.88), two trials, 208 women, low-quality evidence.
• IUI with clomiphene or letrozole versus expectant management, in couples with a low predicted chance of natural conception: probably better. OR 4.48 (95% CI 2.00 to 10.01), one trial, 201 women, moderate-quality evidence. Against a 9% live birth rate with expectant management, this suggests 17% to 50%.
• IUI in a stimulated cycle versus IUI in a natural cycle: may be better. OR 2.07 (95% CI 1.22 to 3.50), four trials, 396 women, low-quality evidence.

The pattern is consistent: IUI on its own, in couples who might have conceived anyway, has not been shown to add much. IUI combined with ovarian stimulation, in couples with a genuinely low chance of conceiving naturally, is where the evidence is strongest. Cochrane's authors also note the main safety concern with stimulated IUI is multiple pregnancy.

Who it is actually for

NICE NG257 makes IUI a targeted treatment rather than a general first step. It recommends offering 12 cycles of unstimulated IUI before considering IVF for:

• people who are unable to, or would find it very difficult to, have vaginal intercourse because of a clinically diagnosed physical disability or psychosexual problem
• couples where the partner with male reproductive organs has azoospermia, where surgical sperm retrieval is unsuitable or unsuccessful, and who wish to use donor sperm

For people using donor insemination who have not conceived after 6 cycles, NICE recommends a further 6 cycles of unstimulated donor IUI before considering IVF. NG257 also sets the referral trigger for anyone using artificial insemination at 6 cycles rather than the usual year.

The change most worth knowing about is for unexplained fertility problems. NG257 advises trying to conceive for a total of 2 years first, and then, before IVF, to consider up to 4 cycles of IUI with ovarian stimulation using gonadotrophins — or to offer IVF directly. NICE separately recommends against ovarian stimulation as a stand-alone treatment for unexplained fertility problems.

The HFEA lists who IUI is generally not suitable for: blocked fallopian tubes, severe endometriosis, low egg quality or low ovarian reserve, and male factor infertility. Women over 40 may be advised to go to IVF directly.

What a cycle involves

1. Tubal check. IUI depends on at least one open fallopian tube, so this is confirmed first — by ultrasound, X-ray (HSG) or keyhole surgery.
2. Fertility drugs, or not. In a stimulated cycle you take medication to encourage the ovaries to produce eggs; in an unstimulated cycle you do not.
3. Monitoring scans. Ultrasound tracks follicle development.
4. Trigger. A hormone injection triggers ovulation, timing the insemination.
5. Sperm preparation and insemination. The sample is washed and prepared, then passed through a fine catheter into the uterus. The procedure itself takes minutes and does not require anaesthesia.
6. Wait, then test.

Because it is done in a licensed clinic, donor sperm used this way comes with screening, traceability and legal parenthood. NICE explicitly advises people thinking about using donor sperm to have treatment in a licensed fertility clinic for exactly those reasons.

The risks worth understanding

The main risk is multiple pregnancy, and it is not a small consideration — twins and higher-order pregnancies carry higher rates of prematurity, low birth weight and neonatal death. The risk depends on how many follicles develop and whether fertility drugs are used, which is why monitoring scans matter and why a cycle is sometimes cancelled or converted. UK practice has moved sharply on this: the HFEA reports the multiple birth rate across fertility treatment fell to 3.4% in 2023, from around 28% in the early 1990s.

Cochrane's estimates of multiple pregnancy rates in the IUI trials were all imprecise and could not distinguish between arms, so the honest position is that this risk is managed clinically, cycle by cycle, rather than settled by trial data.

What to do next

1. Ask which version is being proposed — unstimulated or stimulated, and with which drug. The evidence differs substantially between them.
2. Ask about your tubes first. If tubal status is unknown, that is the test to do before anything else.
3. Ask for the clinic's live birth rate for IUI in your age band and your indication, not its overall figure.
4. Ask how many cycles are planned before the plan changes. NICE's numbers — 12 unstimulated cycles in the specific indications, 6 further donor IUI cycles, up to 4 stimulated cycles in unexplained infertility — give you something concrete to hold a conversation against.
5. If you are over 40 or have a known tubal or severe male factor problem, ask directly whether IUI is the right treatment at all, or whether it delays a more effective one.

When to seek help

If you are using donor insemination at home or through a clinic and have not conceived after 6 cycles, NICE says that is the point for clinical assessment and investigation of both partners where partner sperm is being used. That threshold is half the usual year and it exists for a reason — do not wait out the full twelve.

Otherwise the standard triggers apply: assessment after 1 year of unprotected vaginal intercourse, referral at first presentation from age 36 or where either partner has a known or suspected cause. Scale of use in the UK gives some sense of proportion: the HFEA recorded around 5,500 donor insemination cycles in 2023, resulting in around 820 babies, with single patients making up 48% of DI patients.