Abstract: We included 15 studies with a total of 1326 women. All included studies compared a vasodilator versus placebo or no treatment. We judged most of these studies as having unclear risk of bias. Overall, the quality of evidence was low to moderate for most outcomes. The main limitations were imprecision due to low numbers of events and participants and risk of bias due to unclear methods of randomisation. Vasodilators probably make little or no difference in rates of live birth compared with placebo or no treatment (RR 1.18, 95% confidence interval (CI) 0.83 to 1.69; three RCTs; N = 350; I² = 0%; moderate-quality evidence) but probably increase overall rates of side effects including headache and tachycardia (RR 2.35, 95% CI 1.51 to 3.66; four RCTs; N = 418; I² = 0%; moderate-quality evidence). Evidence suggests that if 236 per 1000 women achieve live birth with placebo or no treatment, then between 196 and 398 per 1000 will do so with the use of vasodilators. Compared with placebo or no treatment, vasodilators may slightly improve clinical pregnancy rates (RR 1.45, 95% CI 1.19 to 1.77; 11 RCTs; N = 1054; I² = 6%; low-quality evidence). Vasodilators probably make little or no difference in rates of multiple gestation (RR 1.15, 95% CI 0.55 to 2.42; three RCTs; N = 370; I² = 0%; low-quality evidence), miscarriage (RR 0.83, 95% CI 0.37 to 1.86; three RCTs; N = 350; I² = 0%; low-quality evidence), or ectopic pregnancy (RR 1.48, 95% CI 0.25 to 8.69; two RCTs; N = 250; I² = 5%; low-quality evidence). All studies found benefit for endometrial thickening, but reported effects varied (I² = 92%) and ranged from a mean difference of 0.80 higher (95% CI 0.18 to 1.42) to 3.57 higher (95% CI 3.01 to 4.13) with very low-quality evidence, so we are uncertain how to interpret these results. Evidence was insufficient to show whether vasodilators increase the live birth rate in women undergoing fertility treatment. However, low-quality evidence suggests that vasodilators may slightly increase clinical pregnancy rates. Moderate-quality evidence shows that vasodilators increase overall side effects in comparison with placebo or no treatment. Adequately powered studies are needed so that each treatment can be evaluated more accurately.

Plain-language summary: We found 15 randomised controlled trials (a type of experiment in which people are randomly allocated to one or more treatment groups) that compared the use of vasodilators versus placebo or no treatment in a total of 1326 women undergoing fertility treatment. The evidence is current to October 2017. Only three of the included studies reported live birth rates. Overall, vasodilators probably make little or no difference in rates of live birth. Moderate-quality evidence shows that vasodilators probably increase overall rates of side effects (including headache and tachycardia (faster than normal heartbeat)) in comparison with placebo or no treatment. However, low-quality evidence suggests that vasodilators may increase the chance of becoming pregnant. The evidence is of low to moderate quality. More research is needed (one study is ongoing and will be incorporated into this review in a subsequent update).