'conception', lit. 'descent', designates the appearance of the embryo in the mother's womb, i.e. the beginning of the birth process (jāti, q.v.).
"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 karma energy) is ready; under these 3 circumstances does the embryo appear" (M. 38).