Two things.
1. These are population studies which result in statistical averages. Individual results may vary wildly.
2. Remember he said that higher fetal T exposure
helps realize the genetic potential of inherited penile size. If potential genetic size isn't big then high fetal T won't create a big penis--just the largest genetics allow.
In other words a low 2D-4D ratio alone doesn't mean a guy will have a big one just the biggest his inherited genes allow. You still won't know until you get into his pants. Sorry.
Biology rules.
BTW the male fetus doesn't get testosterone from his mother. His own testes create it beginning at ten weeks. Before that his fetus developed as a female which is why males have nipples. This is true for all mammalian species. If you've ever rubbed the furry belly of your pet male cat or dog you know he has as many nipples as females of his species.
And the penis stretching metric? Total cringe.