Pregnant women are advised not to handle broken tablets of Propecia (finasteride) because this medication can cause birth defects in male babies. Finasteride is an enzyme inhibitor that blocks the effects of 5-alpha-reductase 2, which inhibits the conversion of testosterone to 5-dihydrotestosterone and 5-androstanediol. In men with benign prostatic hyperplasia, these two androgens act on the prostate and makes it hypertrophy. Propecia stops this from happening. When a pregnant woman absorbs finasteride through her skin by handling broken tablets, the drug inhibits 5-alpha-reductase 2 in the male fetus. When these boys are born, the external genitalia are female; the internal genitalia are male. They are genotypically 46,XY males, but phenotypically female. This condition is generally not recognized right away and the child is raised as a girl. At puberty, androgen levels rise and the "female" child suddenly develops a normal-sized, fully function penis. Prescribing information for Propecia: http://www.merck.com/product/usa/pi_circulars/p/propecia/propecia_pi.pdf eMedicine article on androgen excess: http://www.emedicine.com/med/topic3489.htm "The Role of Androgens in Male Gender Role Behavior" (J.D. Wilson): http://www.transgenderzone.com/library/ae/fulltext/2.htm