ObjectiveTo compare success rates, associated risks and cost-effectiveness between intrauterine insemination (IUI) and in vitro fertilisation (IVF).DesignRetrospective observational study.SettingThe UK from 2012 to 2016.ParticipantsData from Human Fertilisation and Embryology Authority’s freedom of information request for 2012–2016 for IVF/ICSI (intracytoplasmic sperm injection)and IUI as practiced in 319 105 IVF/ICSI and 30 669 IUI cycles. Direct-cost calculations for maternal and neonatal expenditure per live birth (LB) was constructed using the cost of multiple birth model, with inflation-adjusted Bank of England index-linked data. A second direct-cost analysis evaluating the incremental cost-effective ratio (ICER) was modelled using the 2016 national mean (baseline) IVF and IUI success rates.Outcome measuresLB, risks from IVF and IUI, and costs to gain 1 LB.ResultsThis largest comprehensive analysis integrating success, risks and costs at a national level shows IUI is safer and more cost-effective than IVF treatment.IVF LB/cycle success was significantly better than IUI at 26.96% versus 11.49% (p<0.001) but the IUI success is much closer to IVF at 2.35:1, than previously considered. IVF remains a significant source of multiple gestation pregnancy (MGP) compared with IUI (RR (Relative Risk): 1.45 (1.31 to 1.60), p<0.001) as was the rate of twins (RR: 1.58, p<0.001).In 2016, IVF maternal and neonatal cost was £115 082 017 compared with £2 940 196 for IUI and this MGP-related perinatal cost is absorbed by the National Health Services. At baseline tariffs and success rates IUI was £42 558 cheaper than IVF to deliver 1LB with enhanced benefits with small improvements in IUI. Reliable levels of IVF-related MGP, OHSS (ovarian hyperstimulation syndrome), fetal reductions and terminations are revealed.ConclusionIUI success rates are much closer to IVF than previously reported, more cost-effective in delivering 1 LB, and associated with lower risk of complications for maternal and neonatal complications. It is prudent to offer IUI before IVF nationally.