A Trial to Compare Efficacy and Safety of Follitropin Delta Versus Placebo (Inactive Treatment) in the Treatment of Men With Idiopathic Infertility (Unexplained Reduction of Semen Quality) (ADAM)

Inclusion Criteria: * History of infertility with current partner at randomisation must be 12-60 months if current partner is aged \<35 years or 6-60 months if current partner is aged 35-38 years. * Men between the ages of 18 and 50 years. * Total sperm count 5-39 million at screening; confirmed by two consecutive samples taken ≥2 weeks apart before randomization. * Total sperm motility of ≥10% at screening; confirmed by two samples taken ≥2 weeks apart before randomisation. If a semen sample has been taken within 3 months prior to screening and been analysed at an andrology laboratory, it can be included as the first of the two semen samples at screening. * Semen volume ≥1.4 mL at screening; confirmed by two consecutive samples taken ≥2 weeks apart before randomization. * Serum follicle-stimulating hormone (FSH) levels of 2-12.0 IU/L (measured at central laboratory) at screening) * Serum luteinising hormone (LH) levels of 1.2-7.5 IU/L (measured at central laboratory) at screening. * Serum total testosterone levels of ≥300 ng/dL (equals ≥10.4 nmol/L; measured at central laboratory) at screening. * Agree to have regular intercourse with current female partner with the intent of spontaneous conception within 9 months from randomization. * Agree to provide information on female partner's positive urine pregnancy test(s) and documentation of ultrasound(s), delivery, and neonatal/infant health. Current partner fulfilling the criteria below: * Pre-menopausal woman between the ages of 18 and 38 years (both inclusive) at the time of randomisation of male participant. * Regular menstrual cycles of 21-35 days. * No history or current condition of pelvic inflammatory disease, endometriosis stage II-IV by definite or empirical diagnosis, or tubal ligation. * Agree not to obtain infertility treatment outside of this trial for 6 months from randomization of male subject. Exclusion Criteria: * Previous FSH treatment for ≥4 months not leading to conception. * Past or current use of finasteride within 3 months prior to screening. * Any history of anatomical disorder of the pituitary gland or testes. * Any structural abnormalities of the vas deferens (unilateral or bilateral) at screening. * Any known, clinically significant, systemic disease in addition to the trial indication that might negatively impact fertility. * Known history or presence of clinical varicocele (subclinical and Grade 1 varicocele are acceptable). * Known history of cryptorchidism, testicular torsion, or orchitis. * Known abnormal karyotype (including Y-chromosome microdeletion). * Current or past treatment of urogenital (kidney, bladder, testicular, or prostate) cancer as well as history of chemo- or radiotherapy that can have impact on testes. * Any known uncontrolled non-gonadal endocrinopathies (thyroid, adrenal, pituitary disorders). * Administration of hormonal preparations, agents known to impair testicular function or affect sex hormone secretion, and known or suspected teratogens within 3 months prior to screening. Administration of anabolic steroids within 12 months prior to screening.