43 H2Oil2 study protocol 3 BACKGROUND Infertility, defined as the inability to conceive within 12 months of unprotected intercourse, was estimated to effect approximately 48.5 million couples worldwide in 2010 (1, 2). Fertility work-up includes the medical investigation into the various causes of male and female infertility that have been identified. With male factor infertility referring to the (relative) absence of functioning spermatozoa, female infertility can refer to ovulation disorders, diminished ovarian reserve, tubal factor infertility, and uterine factor infertility (3). Ovulation disorders can have various causes, of which polycystic ovary syndrome (PCOS) is the most prevalent (4). Diminished ovarian reserve plays an increasing role in infertility as couples have been postponing their wish to conceive over recent decades (5). Tubal factor infertility can be caused by current or past pelvic inflammatory disease such as an infection with Chlamydia trachomatis, previous pelvic surgery, peritonitis or endometriosis (6, 7). Uterine factor infertility can consist of anatomical anomalies, such as congenital uterine anomalies, or intrauterine pathology such as polyps, myomas or adhesions (8). In up to 30% of couples, the fertility work-up shows no abnormalities and this is classified as unexplained infertility (9, 10). Part of the fertility work-up is assessing the risk for tubal pathology and if indicated, a tubal patency test can be performed. Hysterosalpingography (HSG) is traditionally used as first choice tubal patency test to rule out tubal pathology (3, 4). Although HSG was initially introduced as a diagnostic test, therapeutic effects of tubal flushing, especially with oil-based contrast, have been studied extensively (11-14). The most recent review (14) included six studies comparing HSG with oil-based contrast and water-based contrast. Three of these studies reported on live birth but meta-analysis could not be performed due to heterogeneity of the population. The largest study among these randomized controlled trials is the H2Oil study (15). This study was conducted to investigate the difference in ongoing pregnancy rates between tubal flushing during HSG with the use of oilbasedand water-based contrast, in couples with unexplained or mild male infertility. This study excluded women aged 39 years or older, women with ovulation disorders, and women who had a high risk for tubal pathology. The H2Oil study showed a significant increase in ongoing pregnancies as well as live births within 6 months after HSG with oil-based contrast when compared to HSG with water-based contrast (relative risk (RR) 1.37, 95% confidence interval (CI) 1.16–1.61; P < 0.001 for ongoing pregnancy and RR 1.38, 95% CI 1.17 to 1.64; P < 0.001 for live birth) (15). The long term follow-up of this study demonstrated that the fertility enhancing effect of oil-based contrast is still present five years after HSG (cumulative ongoing pregnancy rates 80.0% after oil-based contrast, 75.0% after water-based contrast, RR
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