*amniotic testosterone levels, apparently, and the ratio is supposedly unaffected by post-birth levels.

"The 2D:4D digit ratio is the ratio of the length of the second (index) finger and the fourth (ring) finger. This ratio is, on average, different in men and women. (Men tend to have longer ring fingers relative to their index fingers, while women’s index fingers are about the same length as, or slightly longer than, their ring fingers.) The idea is that prenatal testosterone levels influence digit ratio."