To evaluate the safety profile and the number of usable blastocysts on day 5 and on day 6 after treatment with an individualized dosing regimen of a follitropin delta and highly purified human menopausal gonadotropin (HP-hMG) for controlled ovarian stimulation. Multicenter, open label, exploratory study. Reproductive medicine clinics. A total of 110 patients (aged 18-40 years). Follitropin delta coadministered with HP-hMG, with follitropin delta dose fixed according to an established algorithm and HP-hMG dose at 75 IU when the follitropin delta starting dosage was <12 μg; 150 IU when follitropin delta dosage was 12 μg and weight <100 kg, and 225 IU when follitropin delta dosage was 12 μg and weight ≥100 kg (dosage adjustments confined to HP-hMG only). Mean number of good-quality blastocysts obtained at day 5 and day 6 as well as the proportion of women with ovarian hyperstimulation syndrome (OHSS). A cohort study was compared with the follitropin delta group from the Evidence-based Stimulation Trial with Human Recombinant Follicle-Stimulating Hormone in Europe and Rest of World 1 (ESTHER-1) study. Even when stratified by age, a statistically significantly higher mean in the number of oocytes retrieved and number of good-quality blastocysts was observed in this study compared with the ESTHER-1 trial in which follitropin delta was used alone. The rate of patients triggered with a gonadotropin-releasing hormone agonist was statistically significantly higher in our Menopur and Rekovelle Combined Study (MARCS) cohort (43%) when compared with the rates reported in the follitropin delta cohort in the ESTHER-1 study (2.3%). Incidence of any grade of OHSS was 9.3% in the present study compared to 2.6% in follitropin delta group from ESTHER-1 trial. No cases of moderate or severe OHSS were observed in our study compared with 1.4% in the follitropin delta group of ESTHER-1. Optimizing the ovarian response during invitro fertilization employing a mixed protocol of individualized dosing of follitropin delta and HP-hMG resulted in a statistically significant number of usable blastocysts on days 5 and 6 with an increased risk of mild OHSS, which did not require medical intervention or hospitalization. NCT03483545.