Relation between headaches from oral contraceptives and development of endometrial arterioles

The incidence of headaches during the first year on oral cotraceptives corrleates closely with the incidence of well-developed arterioles in endometrial biopsy specimens. This relationship appears to depend on the ratio between progestogenic and oestrogenic components of the pill as well as the particular steroids. Therefore endometrial biopsies are useful for assessing the vascular response to an oral contraceptive and also provide a short cut to clinical evaluation of new formulations.