Progestin Induced Virilization (PIV), discovered around 1970, is one form/cause of intersexuality in which a female (karyotype 46XX) fetus develops somewhat more masculine primary sexual characteristics than the norm as a result of in utero exposure to inflated levels of the hormone androgen -- a metabolic product of progestins. Extreme PIV is commonly diagnosed in newborns, but many cases are not noted until the onset of puberty when abnormalities become more important to the patient and may begin to change. Certainly, many cases of PIV have remained undiagnosed and the frequency of the abnormality remains unknown.
The degree of virilization experienced ranges widely. In many cases it consists primarily of an enlarged clitoris (clitoral hypertrophy), but may include the full development of a phallus and the fusion of the labia. The female gonads (ovaries) develop normally and the uterine tract is largely unharmed -- in the most extreme cases the vagina or cervix may never develop.
There is a broad set of medical protocols for surgically correcting Intersex abnormalities (many of which have generated tragic outcomes for the patients involved, and virtually all of which -- in the opinion of this author, are barbaric). Because the PIV alterations occur only prior to birth, the ovary-related endocrinological systems remain intact. As a result, natural feminization is typical during puberty.
Progestins entered the system of these fetuses because it was administered to pregnant women during the 1950s and ‘60s as Provera (Medroxyprogesterone Acetate) to prevent miscarriage. Sadly, since the days when this drug was routinely used like this, studies have found no such preventative value.