Okkanti: 'conception', lit. 'descent', designates the appearance of the embryo in the mother's womb, i.e. the beginning of the birth process jāti.;Through the concurrence of 3 circumstances arises the embryo. When father and mother have united,... and the mother has her time, and the 'genius' metaphorically for the kamma energy is ready; under these 3 circumstances does the embryo appear; M. 38.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.