A Window That Closes

The male path doesn't just need testosterone — it needs it on schedule. Between roughly weeks 8 and 14 of pregnancy, a surge of testosterone drives the cascade of male development. The fetal testes don't start releasing testosterone until about week 8, and the signal has to arrive during that window. Outside it, the same hormone has a weaker effect, and the body keeps drifting toward the default female anatomy.

So development depends on three things lining up: the right gene, the right hormone, and the right time. And as we saw, even perfect timing fails if the cells can't read the signal.