45 H2Oil2 study protocol 3 highest in ranking. Women will be excluded if they have an endocrine disorder known to decrease natural pregnancy chances (e.g. diabetes, unregulated hypothyroidism or hyperthyroidism), iodine contrast medium allergy or a male partner with severe infertility (a pre-washed total motile sperm count below three million sperm per millilitre). Randomization and blinding Infertile couples will be screened in the outpatient clinic by their attending physician. Eligible women will be informed by a dedicated research nurse or physician in their centre. Women who give written consent will be randomized for HSG with oil-based contrast (intervention group) or with water-based contrast (control group) in a 1:1 ratio, using a permuted block design with block sizes varying from 4–8 cases. Randomization will be performed using the web-based program Castor EDC (Castor Electronic Data Capture, Ciwit BV, Amsterdam, the Netherlands), and stratified according to centres and by infertility diagnosis (age 39 years or older, ovulation disorder, and high risk for tubal pathology). The trial is not blinded with respect to participants and health care professionals since the allocation will be evident in further fertility workup. Oil-based contrast has a higher iodine concentration than water-based contrast and together with its hydrophobic qualities this makes oil-based contrast easily distinguishable from waterbased contrast on X-ray or fluoroscopy images (17). The primary outcome is objective, and therefore we do not expect that lack of blinding will influence the findings. Intervention The HSG procedure will be performed after cessation of menstrual bleeding or after progesterone-induced vaginal bleeding in case of anovulation. With use of a cervical vacuum cup, a metal cannula (hysterophore), an acorn cannula or an HSG balloon catheter, the iodinated oil- or water-based contrast medium will be infused into the uterine cavity and several radiographs will be taken to visualize the uterine cavity and Fallopian tubes according to local protocols. The procedure will be discontinued if signs of intravasation are visible on the radiographs, as intravasation of specifically oil-based contrast might lead to oil-embolisms, a known and potentially dangerous complication (18). The results of the HSG will describe whether the Fallopian tubes are patent and whether there are any visual abnormalities in the cervix, uterine cavity, Fallopian tubes or peritoneal cavity. In the intervention arm, the HSG will be performed with a maximum of 15 millilitre of oil-based contrast (Lipiodol Ultra Fluid®, Guerbet, Villepinte, France) to minimize the chance of (temporary) hypo- or hyperthyroidism (18). In the comparator arm, the HSG will be performed with water-based contrast medium (iodixanol, Visipaque®, General Electric Healthcare, Buc, France), for which no maximum dosage is advised. The batch number and expiration date of the used flasks of contrast medium will be reported for the purpose of drug accountability.
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