The REWAG study (Real-world Evaluation of Women undergoing Agonist protocol with Follitropin delta) is a non-interventional observational study conducted across several fertility centers in Germany. The goal of this study is to evaluate how well a personalized dosing regimen of a hormone called Follitropin delta works in women undergoing controlled ovarian stimulation (COS) for in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) using a long GnRH agonist protocol. Follitropin delta is a recombinant follicle-stimulating hormone (rFSH) that allows for individualized dosing based on a woman's weight and a blood marker called anti-Müllerian hormone (AMH). This personalized approach may help to better balance the stimulation of the ovaries, aiming to reduce the risk of overstimulation (OHSS) while still achieving good treatment outcomes. The study will include approximately 350 women who are undergoing routine IVF/ICSI treatment. No experimental drugs or procedures are involved. All treatment decisions remain the responsibility of the attending physicians and follow standard clinical practice. Data will be collected only from routine visits and medical documentation, with no additional interventions required for participation. Researchers will analyze outcomes such as the number of eggs retrieved, pregnancy rates, treatment cancellations, and any side effects. The study will also look at whether certain patient characteristics can predict how well the treatment works or whether complications may arise. Participation is voluntary. All personal data will be pseudonymized and handled according to strict data protection regulations (GDPR). Results will help to improve understanding of how personalized hormone dosing performs in real-life settings and may support more tailored and effective fertility treatments in the future.