Strictly speaking, conception is after a spermatozoa penetrates the ovum and then becomes a zygote which is after the fused gametes begin dividing into cells. Unless the fused gametes actually divide into separate cells, it's not viable so it's safe to say conception never occurred.
It can also be said that conception is when a viable zygote implants in the uterus marking the very start of pregnancy.
I think the former is closer to the definition of conception.