Until 12-14 hours after mating, the mouse vagina is "blocked" by a so-called "plug". It's dried up liquid that came with the sperm. So, you put the females in a cage with the males, and 11-12 hours later, you check for plugs. Females with a plug can be uses 2 days later for the embryo transfer.
We usually inject the eggs at the blastocyst stage or slighty before, about 3 days after the microinjection of the components.
I didn't know about the plug thing , i do understand now ! anyway looking forward to see your newborns mice :D