During two none subsequent natural menstrual cycles healthy female participants will be subjected every second day to blood sampling and 3D-ultrasound examinations. Together with four other key hormones the concentration of AMH (anti-Muellerian Hormone) will be measured in the serum.

During two none subsequent natural menstrual cycles healthy female participants will be subjected every second day to blood sampling and 3D-ultrasound examinations. Together with four other key hormones the concentration of AMH will be measured in the serum. Half of the serum samples will immediately be subjected to hormone analysis whereas the other half will be stored frozen at -80 °C. After conclusion of the study the frozen samples will be subjected to re-measurment again a reference laboratory. The aim is to study the stability of the AMH Elecsys Assay under both conditions. The participants will be health women aged between 18 and 40 years. They will be grouped into a younger and an older group. They will be observed during one menstrual cycle, then they will remain unobserved during a second menstrual cycle, and then again they will be observed during a third menstrual cycle.

Inclusion Criteria:

• regular menstrual cycles, between 24 and 32 d.
• body mass index between 24 and 32 kg/m2
• no HIV, no Hepatitis B and C
• no smoking

Exclusion Criteria:

• intake of reproductive hormones, including birth control pill
• pregnancy or breastfeeding
• infertility
• polycystic ovary syndrome

Only women are recruited.

healthy female women with regular menstrual cycles.