Well firstly the don't get pregnant.
Mature females swell up with eggs and if you have a swollen cory then it's odds on it's carrying eggs.
If you have a couple of males then breeding is quite possible, when ready the male nudge at the females and if the become very active 2 fish positioning to form a "T" shape this is typical breeding behaviour.
A simple trigger to trigger them to spawn is a large cool water change, change 40-50 of the water and aim for the resualting mix to be 4-5oC cooler and dont allow it to warm to quickly can help make them spawn with in 2-3 days if all is well.