Ovarian hyper stimulation syndrome (OHSS) is a potentially serious complication of ovarian stimulation in IVF (In Vitro Fertilisation). The main objective of this study is to describe the incidence of moderate or severe forms of OHSS in women between 18 and 36 years of age treated with HP-hMG.

Inclusion Criteria:

• Women who present infertility of more than one year and are candidates for a first or second IVF cycle with or without ICSI for whom HP-hMG is prescribed for COS.
• Absence of infertility treatment during the last 3 months before enrolment.
• Age: from 18 to 36 years old.
• BMI between 18 and 30 kg / m².
• Presence of both ovaries accessible to puncture and absence of ovarian or uterine abnormalities.
• Male or female infertility.
• Ovarian stimulation by HP-hMG with pituitary desensitization by a GnRH agonist or antagonist.
• Normal ovarian reserve according to physician habitual evaluation.
• Consent to participate of the no-interventional study and signature of the patients' information sheet.

Exclusion Criteria:

• Presence of a chronic disease, cancer or endocrine disease that could potentially influence the results of the stimulation or that represents a contraindication for ovarian stimulation.
• Known endometriosis grade III or IV.
• Contraindication to the use of gonadotropins or current pregnancy diagnosed by the clinician.
• Recurrent miscarriages, known genetic disease of one of the partners or indication of a preimplantation genetic diagnosis (PGD).
• Smoking over than 10 cigarettes / day.´
• Participation in an interventional study at the time of inclusion.
• Known poor ovarian response in a previous cycle of stimulation (number of oocyte collected ≤ 3, and / or more than 2 previous IVF/ICSI cycles and / or abnormal result in ovarian reserve test (AMH < 1 ng/ml with Immunotech equipment or AMH<0.7 ng/ml with DSL equipment).

Women from 18 to 36 years old treated by HP-hMG for controlled ovarian stimulation (COS) for a first or second cycle of IVF / ICSI.