In this prospective observational study, the effect of timing of insertion (early/late follicular; early/late luteal) on ovarian cyst development in women receiving an LNG-IUD will be assessed ultrasonographically at months 0, 3, 6, and 12. Secondary outcomes include cyst size/structure, bleeding pattern, pain, and other adverse events.

The menstrual cycle will be divided into four subphases, and the LNG-IUD will be placed according to these phases.Participants will be assessed initially with a gynecological examination, ultrasound, and hormone profile (FSH, LH, TSH, prolactin, estradiol, progesterone); they will be followed up with ultrasound at months 0, 3, 6, and 12.There is no randomization or blinding (observational design).Data analysis will be conducted using chi-square/Fisher, with Yates correction if necessary, and Cramer's V/phi as the effect size (p<0.05/0.01).

Inclusion Criteria:

• Reproductive age: 18-45 years
• Patients with indications for LNG-IUD (levonorgestrel-containing intrauterine device) insertion (abnormal uterine bleeding, menorrhagia, dysmenorrhea, endometriosis, adenomyosis, hormonally induced dysfunctional uterine bleeding, endometrial protection in women receiving postmenopausal hormone replacement therapy, and endouterine hyperplasia without atypia)
• Patients without contraindications for LNG-IUD (no suspected pregnancy, no serious uterine anomalies (Müllerian anomalies), no active pelvic infection, no history of serious liver disease or breast cancer, and no anatomical or clinical contraindications to IUD insertion) will be included.
• They must not have used oral contraceptives for at least 3 months.
• The previous IUD must have been removed at least 1 year prior.
• Those who signed the informed consent form and agreed to participate voluntarily will be included in the study.
• Those who have not used steroidal anti-inflammatory drugs within 24 hours of insertion of a levonorgestrel-containing IUD will be included in the study.

Exclusion Criteria:

• Pregnancy history or current pregnancy
• Patients using hormone therapy
• Patients with adnexal masses
• Patients with severe systemic diseases (uncontrolled diabetes, advanced cardiovascular disease, active malignancies (especially hormone-sensitive ones), advanced liver or kidney failure, systemic connective tissue diseases)
• Patients with contraindications to the levonorgestrel IUD (suspected pregnancy, severe uterine anomaly, active pelvic infection, severe liver disease, or a history of breast cancer, anatomical or clinical obstacles to intrauterine device insertion)
• Patients with psychological disabilities
• Patients who refuse to sign the informed consent form
• Patients who cannot be followed up will not be included in the study.

The study population will consist of women aged 18 to 45 years who are clinically indicated for levonorgestrel-releasing intrauterine device (LNG-IUD) insertion for contraception or therapeutic purposes. Participants will be recruited from the outpatient gynecology clinic of SBÜ Gaziosmanpaşa Training and Research Hospital.

All participants will have regular menstrual cycles and no contraindications to LNG-IUD use. They will be assigned to one of four observational cohorts (early follicular, late follicular, early luteal, or late luteal phase) according to the menstrual phase at the time of insertion.