To investigate the levels of Relaxin in different types of endometrial preparation for a frozen embryo transfer cycle. Up to now it is unclear, whether ovulation with a functional corpus luteum (CL) will be present in a natural proliferative phase (NPP) endometrial preparation approach.

The last years have seen a distinct shift from fresh (fresh ET) to frozen embryo transfer (FET) cycles. Furthermore it became obvious, that the endometrial preparation approach has a significant impact on the course of the pregnancy and that a HRT endometrial preparation approach is associated with a higher risk for bleeding and miscarriages in early pregnancy and for the development of pregnancy induced hypertension and pre-eclampsia later in pregnancy.

These conditions are attributed to the absence of a CL in the HRT approach. Lately a new endometrial preparation approach is discussed and performed, the so called natural proliferative phase protocol, in which the follicle growth is monitored and when the lining is deemed to have a sufficient thickness and the triple lining pattern, exogeneous progesterone is started to induce secretory transformation before spontaneous ovulation. Despite some studies showing, that exogeneous progesterone administration might induce ovulation, it is not clear whether there will be a CL, producing Relaxin, in the NPP approach.

Inclusion Criteria:

• regular menstrual cycles

Exclusion Criteria:

• abnormal findings of the ovaries (e.g endometriosis)
• Intake of any hormones within 3 months of study start
• desire to become pregnant

regular menstrual cycles

- healthy volunteers with regular cycles

on request