Abstract: We included four studies, with a total of 522 women, in the review. One of these studies did not report outcomes per woman randomised, and so was not included in formal analysis. Three studies investigated 10,000 units hCG priming compared to no priming. One study investigated 20,000 units hCG compared to 10,000 units hCG priming. Three studies only included women with PCOS (N = 122), while this was an exclusion criteria in the fourth study (N = 400). We rated all four studies as having an unclear risk of bias in more than one of the seven domains assessed. The quality of the evidence was low, the main limitations being lack of blinding and imprecision. When 10,000 units hCG priming was compared to no priming, we found no evidence of a difference in the live birth rates per woman randomised (OR 0.65, 95% confidence intervals (CI) 0.24 to 1.74; one RCT; N = 82; low quality evidence); miscarriage rate (OR 0.60, 95% CI 0.21 to 1.72; two RCTs; N = 282; I² statistic = 21%; low quality evidence), or clinical pregnancy rate (OR 0.52, 95% CI 0.26 to 1.03; two RCTs, N = 282, I² statistic = 0%, low quality evidence). Though inconclusive, our findings suggested that hCG may be associated with a reduction in clinical pregnancy rates; 22% of women who received no priming achieved pregnancy, while between 7% and 23% of women who received hCG priming did so. The study comparing 20,000 units hCG with 10,000 units hCG did not report sufficient data to enable us to calculate odds ratios. No studies reported on adverse events (other than miscarriage) or drug reactions. This review found no conclusive evidence that hCG priming had an effect on live birth, pregnancy, or miscarriage rates in IVM. There was low quality evidence that suggested that hCG priming may reduce clinical pregnancy rates, however, these findings were limited by the small number of data included. As no data were available on adverse events (other than miscarriage) or on drug reactions, we could not adequately assess the safety of hCG priming. We need further evidence from well-designed RCTs before we can come to definitive conclusions about the role of hCG priming, and the optimal dose and timing.

Plain-language summary: This review included four randomised controlled trials, with a total of 522 women. One study investigated the use of 20,000 units hCG priming compared to 10,000 units. The remaining studies investigated 10,000 units hCG priming compared to no priming. The main outcomes were live birth rate and miscarriage rate per woman randomised. Evidence published up to 29 August 2016 was examined. Only one study reported the main outcome of live birth per woman randomised; two studies reported clinical pregnancy. We found no certain evidence of a difference between 10,000 units hCG priming and no priming. However, there was some low quality evidence to suggest that hCG may be associated with a reduction in pregnancy rates; 22% of women who received no priming achieved pregnancy, while between 7% and 23% of women who received hCG priming did so. Two studies reported miscarriage rate per woman randomised, and found no evidence of a difference between 10,000 units hCG priming and no priming. No studies reported on adverse events (other than miscarriage) or drug reactions. Overall, there was insufficient evidence to draw any definite conclusions on the use of hCG priming in IVM. Further randomised trials are necessary, in particular, focusing on women with PCOS. The quality of the evidence was low, the main limitations being imprecision (random error) and lack of blinding (the process in which participant and assessor are prevented from knowing which intervention has been received).