The Compendium of Bheḷa 4.3.10

4.3.10

tatra śuklottare bhāge pumān raktottare ' ṅganā | anenaiva ca kalpena yamakeṣv api nirdiśet ||

There, in the part where semen is greater, a male [arises]; where blood is greater, a female; / according to this very rule one should also determine [the sex] in twins.