25 Duration of the fertility enhancing effect of HSG with oil-based contrast 2 INTRODUCTION Hysterosalpingography (HSG) is a commonly applied tubal patency test during fertility workup (1, 2). Although it was first introduced as a diagnostic test, therapeutic effects have been debated in studies for many years, especially regarding HSG with use of oilbased contrast (3). In 2017, a multicentre randomized controlled trial (RCT) (under the name of the H2Oil study) showed that HSG using oil-based contrast resulted in a 10% higher absolute ongoing pregnancy rate within 6 months compared with the use of water-based contrast (relative risk 1.37, 95% CI 1.16–1.61) (4). Two subsequent meta-analyses confirmed these findings (5, 6). The most recent meta-analysis aimed to evaluate the long-term effects of tubal flushing; however, only three studies reported a follow-up of more than 12 months, so no definitive conclusions could be drawn (6). This emphasized the need for long-term follow-up studies. Recently, the long-term reproductive outcomes of HSG with oil-based versus water-based contrast have been published (under the name of the H2Oil follow-up study). Over a 5-year follow-up period, HSG with oil-based contrast during fertility workup resulted in more ongoing pregnancies, more live births and a shorter time to pregnancy compared with HSG with water-based contrast (7). However, it remained uncertain whether the 5-year effect was explained by the initial effect of oil-based contrast immediately after HSG or whether the effect was long(er) lasting. Exploring the duration of this fertilityenhancing effect might provide more information on the mechanism of effect of oilbased contrast. To date, this has remained unclear. Several potential mechanisms have been suggested. They can be categorized according to their location of action: the fallopian tube, the endometrium and the peritoneum. First, tubal flushing, i.e. mechanical flushing of debris or mucus plugs or unblocking of peritubal adhesions, can clear passage of otherwise normal fallopian tubes (8). Second, uterine bathing with oil-based contrast can enhance endometrial receptivity. Oil-based contrast is produced from poppy seed and contains opium alkaloids, which potentially interact with opioid receptors in the endometrium (9) or through alterations of the uterine immune response (10). A third potential mechanism is that oil-based contrast reduces peritoneal macrophage phagocytosis and macrophage adherence, by forming an oily layer over the macrophages changing their shape and surface configuration (11). Previous studies showed that sperm phagocytosis is inhibited in vitro by oil-based contrast (12, 13).
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