Inclusion Criteria:  Women, aged between 18 and 45 years premenopausal (women, who had menses at any time in the preceding 24 consecutive months) BMI < 25 kg/m² Ability to understand and voluntarily sign an informed consent document prior to any study related procedures  Exclusion Criteria:  Type 1 or Type 2 diabetes Intake of medication which may affect prolactin levels (e.g. Metoclopramide, Methyldopa, opiates, cimetidine), except birth control pill/hormonal contraception known hyperprolactinemia or hyperthyroidism any other serious illness that might impact the results as judged by the investigator (e.g. malignant disease, coronary heart disease) Pregnant / breastfeeding women (weaning at least 6 month ago) plastic breast surgery (with detachment of milk ducts) allergy to BPA-free plastic