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The study protocol was approved by the Medical Ethical Committee (G17.112). The IVF database, with prospectively collected data from cycles in the IVF laboratory of the Leiden University Medical Centre (LUMC) was analysed. The data was collected from both regional hospitals with transport service and the LUMC itself. So all IVF procedures took place at this laboratory.

Inclusion criteria in the control group were patients with tubal disease or unexplained infertility. Only patients with normospermia and fresh embryos (embryos from fresh oocytes, a procedure without cryopreservation) were included in both groups. Normospermia was defined regarding the World Health Organization (WHO), progressive motility of at least 32% and a minimal sperm concentration of 15 million per ml (Cooper et al., 2010). IVF cycles were categorised according to diagnosis. Endometriosis was staged as advised by the revised American Society Reproductive Medicine (r-ASRM, stage I - IV) (1997). If the stage was missing, the patient file was checked for detailed information on classification. The endometriosis diagnosis was either confirmed by laparoscopy or with imaging techniques such as ultrasound (US) or Magnetic Resonance Imaging (MRI). Tubal disease was confirmed by laparoscopy. Unexplained infertility refers to infertility in couples with apparently normal ovarian function, fallopian tubes, uterus, cervix and pelvis and with adequate coital frequency; and normal testicular function, genito- urinary anatomy and a normal ejaculate (Zegers- Hochschild et al., 2017). Only fresh and first cycles were included to reduce the potential bias that is induced by cryopreservation with its effect on the zona pellucida. Exclusion criteria consisted of cryopreserved cycles, ICSI cycles, no puncture due to low response and ovarian hyperstimulation syndrome (OHSS). Furthermore, oncology patients were also excluded. Regarding the controlled ovarian hyperstimulation protocol prior to the IVF procedure, the vast majority in this study period were treated with the short schedule agonist 150IU follitropin-alfa (Gonal-F) from day four. Endometriosis patients were not treated differently compared to the control group.

Baseline characteristics such as age, body mass index (BMI), smoking and alcohol consumption, primary or secondary infertility, previous surgery and pretreatment with gonadotropin-releasing hormone (GnRH) analogues were collected. Data on previous surgery was subdivided into 5 variables: abdominal surgery (including appendectomy, laparotomy, intestinal fistula surgery), reproductive surgery (including (laparoscopic) surgery of the fallopian tubes, uterus or ovaries), hysteroscopic surgery (including adhesiolysis, polypectomy, uterine septum surgery) or curettage, chromopertubation and no surgery.

The primary outcome was fertilisation rate. Fertilisation was considered normal when 2 pronuclei were present 16-20 hours after insemination. Two groups were identified: patients with clinically proven endometriosis and controls. The latter contained patients suitable for IVF without clinical signs of endometriosis.

Secondary outcomes included number of retrieved oocytes, number of embryo transfers (ET) (standard protocol, day 3 transfer), embryo morphology score (according to the number and size of blastomeres and the amount of fragmentation the embryos were assigned to four different quality scores: type 1: equal-sized blastomeres and no fragmentation; type 2: <20% fragmentation; type 3: 20-50% fragmentation; type 4: >50% fragmentation), pregnancy outcome (biochemical pregnancy: increasing bHCG>=50IU/l at 15 days after oocyte retrieval) and ongoing pregnancy (pregnancy with fetal heart rate).

A power calculation was made, based on the data of Komsky et al. (Komsky-Elbaz et al., 2013). To detect a 10 % difference in fertilization rate, which is considered clinically significant with 80% power and a two-sided α of 5%, 159 oocytes were needed in each group. This assumed a per group standard deviation of 31,9 and an analysis with a two-sided t-test with alpha 0.05.

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