When a couple or an individual is told that medical help is needed to conceive, two treatments come up quickly: intrauterine insemination (IUI) and in vitro fertilisation (IVF). They are often described as "step one" and "step two," which can make IUI sound like a lighter version of IVF. In reality they solve different problems. Which one is appropriate depends on the diagnosis, the person's age, how long they have been trying, what has already been tried, and the patient's own priorities. This article explains how each treatment works, what the best available evidence says, and where guidelines agree and differ.
The Core Difference
IUI places prepared sperm into the uterus around the time of ovulation, so fertilisation still has to happen inside the body. The fallopian tubes must be open, and an egg must be released. IVF takes the egg and sperm outside the body: eggs are collected after ovarian stimulation, fertilised in a laboratory, and one or more resulting embryos are transferred to the uterus (or frozen for later). Because fertilisation and early embryo development are observed in the laboratory, IVF can bypass problems that IUI cannot, such as blocked fallopian tubes. It is also a more demanding treatment, with more medication, monitoring and procedures.
What IUI Involves
In IUI, a semen sample is washed and concentrated in the laboratory so that the most motile sperm are separated from the rest. The sample is then placed into the uterine cavity through a thin catheter, a procedure that takes only a few minutes and usually does not need anaesthesia. IUI can be done in a natural cycle, or after medication to stimulate the ovaries (for example tablets such as clomiphene or letrozole, or injectable gonadotrophins) so that one or a few eggs develop. Ultrasound scans, and sometimes blood tests, are used to time the insemination.
IUI can use a partner's sperm or donor sperm. Some people use it because they do not have a male partner or because intercourse is difficult or not possible. Others use it because the cause of infertility is unexplained or mild. Because IUI depends on the body's own fertilisation process, it is generally not expected to help when the fallopian tubes are blocked, when egg numbers or quality are very low, or when sperm parameters are severely reduced. The UK regulator's patient information also notes that IUI is generally less successful per cycle than IVF and that many people need several cycles.
What IVF Involves
IVF usually begins with hormone injections that encourage the ovaries to produce several eggs. Growth is tracked with ultrasound and hormone tests. When the follicles are ready, a trigger injection finalises egg maturation, and the eggs are collected through a short procedure, typically under sedation. In the laboratory, eggs are combined with sperm (conventional IVF) or a single sperm is injected into each mature egg (intracytoplasmic sperm injection, or ICSI). Embryos are grown for several days, and one or more may be transferred to the uterus. Additional embryos of suitable quality can be frozen.
IVF can be used for a much wider range of diagnoses than IUI, including tubal disease, moderate to severe male-factor infertility, some cases of endometriosis, reduced ovarian reserve, and situations where embryo testing for a genetic condition is planned. Its added complexity also brings added considerations: ovarian hyperstimulation syndrome (OHSS), a reaction to ovarian stimulation, can occur, and the egg retrieval and embryo transfer are procedures in their own right. Modern practice has moved strongly toward transferring a single embryo at a time. In the UK, for example, the multiple-birth rate after IVF fell from 14.4% in 2014 to 3.2% in 2024, and single embryo transfers made up 84% of transfers in 2024.
How the Diagnosis Steers the Choice
Doctors generally begin with a basic evaluation: checking ovulation, assessing whether the fallopian tubes are open, analysing semen, and considering age and ovarian reserve. The results shape the options.
- Blocked or damaged fallopian tubes: IUI depends on at least one healthy, open tube, so IVF is usually the relevant treatment.
- Moderate to severe male-factor infertility: IVF, often with ICSI, is more commonly considered, because IUI relies on a sufficient number of motile sperm reaching the egg.
- Ovulation disorders: Ovulation induction, with or without IUI, is often the first approach, and IVF may follow if this does not lead to pregnancy.
- Endometriosis: NICE guidance (2026) says that if conception has not occurred after two years of expectant management or after surgical treatment, clinicians can consider up to four cycles of IUI with ovarian stimulation using gonadotrophins, or offer IVF. It also lists the length of time trying, symptom severity, age, ovarian reserve and any male-factor issues as points to weigh.
- Use of donor sperm without a known fertility problem: IUI is frequently a reasonable first treatment when the person receiving it has no identified female-factor problem.
- Unexplained infertility: This is where the comparison is most debated, and it is covered next.
Unexplained Infertility — What the Evidence Shows
Unexplained infertility means that standard tests are normal and no cause has been found. The 2023 European Society of Human Reproduction and Embryology (ESHRE) guideline defines it for women aged 40 or younger, partly because the diagnosis becomes much less reliable with advancing age.
A 2023 Cochrane systematic review (Sunkara and colleagues) pooled randomised trials comparing IVF with other options in unexplained infertility. Its main findings were as follows:
- Compared with IUI without ovarian stimulation, IVF may improve the live birth rate (odds ratio 2.47, 95% confidence interval 1.19 to 5.12; two trials, 156 women), but the certainty of this evidence was low.
- In women who had not had previous treatment, there may be little or no difference in live birth between one IVF cycle and two to three cycles of IUI with gonadotrophins (odds ratio 1.19, 95% confidence interval 0.87 to 1.61; three trials, 731 women).
- In women who had previously been treated with IUI plus clomiphene, IVF may improve live birth compared with IUI plus gonadotrophins (odds ratio 3.90, 95% confidence interval 2.32 to 6.57; one trial, 280 women).
- Evidence on multiple pregnancy, miscarriage and OHSS was mostly of very low certainty, and the authors advised cautious interpretation overall.
The ESHRE guideline reached a practical position from this evidence. It gives a strong recommendation that IUI with ovarian stimulation be considered first-line treatment for couples with unexplained infertility, and a conditional recommendation that IVF is probably not recommended over IUI with ovarian stimulation. It also states that the decision to use IVF should be individualised according to factors such as age, duration of infertility, previous treatment and previous pregnancy, and that gonadotrophins should be used at a low dose with adequate monitoring to avoid multiple pregnancy and OHSS. It does not recommend ICSI over conventional IVF in this group.
It is worth knowing that guidance is not identical everywhere. The UK regulator's patient information notes that national guidance in the UK has advised against IUI for couples with unexplained infertility, and that women in their late 30s or older may be advised to consider IVF instead. This reflects genuine uncertainty in a field where trials are small, treatments and monitoring have changed over time, and a cycle of IUI and a cycle of IVF are not directly comparable in cost, burden or cumulative chance. When two reputable bodies reach different conclusions, the sensible reading is that both approaches can be reasonable, and that individual circumstances should decide.
Why Age and Time Matter
Age affects both the number and the genetic quality of eggs, and fertility declines with time. A treatment that is reasonable to try for several cycles at 30 may be less appropriate at 40, because each month of unsuccessful treatment is more significant. Guidelines therefore treat age and the duration of infertility as central factors rather than footnotes. The 2023 ESHRE guideline specifically cautions that the accuracy of an "unexplained" label falls with age, so that a more thorough review may be needed when the woman is approaching or beyond 40.
This is also why a "try IUI first" plan usually comes with a defined stopping point. Many clinics agree in advance on a limited number of IUI cycles, after which the plan is reviewed. The UK regulator's patient information, for instance, notes that when several IUI cycles have not worked, further tests may be done and IVF may be suggested.
Safety and Treatment Burden Compared
IUI is simpler, involves fewer procedures, and avoids egg retrieval. Its principal safety concern arises when ovarian stimulation is used: if too many follicles develop, there is a higher risk of twins or higher-order multiples, which carry greater risks for the mother and babies. Careful monitoring and cancelling or converting a cycle when too many follicles develop are standard ways to reduce this risk.
IVF involves more medication, more frequent monitoring and a minor surgical procedure for egg collection, and carries a small risk of OHSS, which clinicians can reduce by individualising the stimulation dose and by choosing how and when embryos are transferred. On the other hand, IVF allows the number of embryos transferred to be controlled directly, which is how multiple pregnancy has been reduced so substantially in settings that favour single embryo transfer. Neither treatment carries zero risk, and both can be emotionally demanding. The World Health Organization's 2025 infertility guideline describes care as a stepped approach driven by test findings and patient preferences, with ongoing psychosocial support recommended throughout.
Questions Worth Discussing With a Fertility Specialist
- What did my tests show about ovulation, the fallopian tubes and semen, and how confident are we in them?
- Given my age and how long we have been trying, how many cycles of IUI would be reasonable before reviewing the plan?
- If IUI with medication is suggested, how will the number of developing follicles be monitored, and when would a cycle be stopped?
- What would make IVF the more appropriate choice for us, and what additional tests or steps would that involve?
- How would single embryo transfer and embryo freezing fit into an IVF plan?
Key Points
IUI and IVF are not interchangeable, and IVF is not simply a stronger version of IUI. IUI is a reasonable option when the tubes are open, ovulation can be achieved and sperm parameters are adequate, as in some cases of unexplained infertility, mild endometriosis, or when donor sperm is used. IVF becomes the more relevant option when tubal disease, significant male-factor infertility or reduced ovarian reserve is present, when age is advanced, or when IUI has not led to pregnancy. In unexplained infertility, the evidence suggests that stimulated IUI and IVF may achieve similar live birth rates in people who have not been treated before, but the evidence is limited, and guidelines differ in emphasis. A decision made with a specialist who has reviewed the full picture, and who explains the likelihood of benefit, the risks and the alternatives, is better than a default pathway. No treatment can promise a pregnancy, and the aim of a good plan is to match the treatment to the cause while limiting avoidable risk and delay.
References
- Sunkara SK, Kamath MS, Pandian Z, Gibreel A, Bhattacharya S. In vitro fertilisation for unexplained subfertility. Cochrane Database of Systematic Reviews, 2023, Issue 9, CD003357. https://www.cochrane.org/evidence/CD003357_vitro-fertilisation-compared-other-options-unexplained-subfertility
- ESHRE Unexplained Infertility Guideline Group. Evidence-based guideline: unexplained infertility. European Society of Human Reproduction and Embryology; published in Human Reproduction, 2023 (doi:10.1093/humrep/dead150). https://www.eshre.eu/Guidelines-and-Legal/Guidelines/Unexplained-infertility
- National Institute for Health and Care Excellence. Fertility problems: assessment and treatment (NG257), management of female factor fertility problems, endometriosis recommendations 1.36.2 and 1.36.3. NICE, 2026. https://www.nice.org.uk/guidance/ng257/chapter/management-of-female-factor-fertility-problems
- Human Fertilisation and Embryology Authority. Intrauterine insemination (IUI). HFEA patient information. https://www.hfea.gov.uk/treatments/explore-all-treatments/intrauterine-insemination-iui/
- Human Fertilisation and Embryology Authority. Fertility treatment 2024: trends and figures. HFEA, 2026. https://www.hfea.gov.uk/about-us/publications/research-and-data/fertility-treatment-2024-trends-and-figures
- World Health Organization. WHO issues first global guideline on infertility. WHO news release, 28 November 2025. https://www.who.int/news/item/28-11-2025-who-issues-first-global-guideline-on-infertility