The Compendium of Caraka 4.2.25

4.2.25

putraṃ tvato liṅgaviparyayeṇa vyāmiśraliṅgā prakṛtiṃ tṛtīyām/
garbhopapattau tu manaḥ striyā yaṃ jantuṃ vrajettatsadṛśaṃ prasūte//

A son [is born] by the opposite signs, and the third nature by mixed signs;
at the conception of the embryo, whatever living being the woman's mind attends to, she gives birth to one resembling that.