At least 26 RCTs and 11 retrospective studies were included.
IUI versus TI and ICI.
Compared with TI, IUI significantly increased the conception rates in natural cycles (6 RCTs; OR 2.5, 95% CI: 1.6, 3.9), while IUI plus COH significantly increased the pregnancy rates (7 RCTs; OR 2.2, 95% CI: 1.4, 3.6). In couples with unexplained fertility, IUI slightly increased the pregnancy rate in comparison with TI or ICI (2 RCTs, 1,691 cycles; OR 2.7, 95% CI: 1.0, 4.4), while IUI plus COH significantly increased the pregnancy rate in comparison with TI (7 RCTs, 980 cycles; OR 1.8, 95% CI: 1.3, 2.6).
IUI using natural cycle versus COH. In couples with male subfertility, COH with CC did not increase the pregnancy rate (2 RCTs; OR 0.78, 95% CI: 0.14, 4.3), whereas COH with gonadotrophins significantly increased the pregnancy rate in comparison with IUI alone (2 RCTs; OR 2.0, 95% CI: 1.1, 3.8). In couples with unexplained infertility, COH using either CC (1 RCT) or gonadotrophins (1 RCT) increased pregnancy when compared with IUI alone; COH/IUI significantly increased conception when compared with IUI alone (1 RCT; OR 1.7, 95% CI: 1.2, 2.6); and COH/IUI significantly increased the pregnancy rate in comparison with ICI (1 RCT; OR 3.2, 95% CI: 2.0, 5.3).
Timing or induction of ovulation and frequency of insemination.
Double IUI at 12 to 43 hours after gonadotrophins seemed to increase the pregnancy rate in comparison with single IUI (3 RCTs; OR 2.3, 95% CI: 1.4, 3.9). The earliest RCT (49 cycles of COH/IUI) found that double insemination increased cycle conception rates using the husband's sperm; the second RCT (169 COH/IUI cycles) found no difference. The most recent RCT (449 COH/IUI cycles using CC and gonadotrophins) found that double insemination 12 and 34 hours after gonadotrophin administration increased pregnancy, compared with single and double insemination at 34 and 60 hours after the administration of gonadotrophins. Five retrospective studies found conflicting results on pregnancy outcome for single versus double insemination frequency.
Factors influencing the pregnancy rates.
Factors within couples: 2 studies found different results. One RCT (258 couples, 963 cycles) found no association between the duration of infertility and conception rate, while one retrospective analysis (260 cycles) found that increasing duration of infertility was associated with a decreased conception rate.
Female factors: the factors associated with the success of IUI treatment included an absence of previous pelvic corrective surgery (1 retrospective analysis, 1,728 cycles), unexplained infertility and anovulation (1 retrospective analysis, 2,473 cycles), and age (6 studies). Endometriosis or tubal problems were associated with decreased success (1 retrospective analysis, 485 cycles).
Male factors: there was a lack of standardisation of the semen analysis. Lower sperm motility counts (3 retrospective analyses) and lower post-wash sperm motility (1 retrospective analysis) were associated with a reduced success of IUI treatment. One meta-analysis found that poor sperm morphology (4% or less) slightly reduced the pregnancy rates in comparison with normal morphology (greater than 4%); the pooled OR (6 studies) was -0.07 (95% CI: -0.11, -0.03).
Sperm processing methods: there was no agreement on the methods used to process sperm for IUI. The pooled data from 2 RCTs (465 cycles, 433 couples) found that density gradient centrifugation led to a borderline increase in the pregnancy rate compared with the wash method; the OR was 1.7 (95% CI: 1.0, 1.9).
Factors associated with multiple pregnancy.
The multiple pregnancy rates for couples undergoing IUI ranged from 14 to 39% (3 studies). The factors associated with multiple pregnancy were peak estradiol level and the number of pre-ovulatory follicles on the day of gonadotrophin (2 studies).