Quantification of the levels of estrogen, or osetrogen, a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal activity: estrone (E1), estradiol (E2), and estriol (E3). Another estrogen called estetrol (E4) is produced only during pregnancy. The trait is estrogen measurement (EFO_0011007, an Experimental Factor Ontology term). Also known as: estrogen levels, oestrogen levels, oestrogen measurement.