Ivermectin paste wiped into the rats mouth works fine I've used it several times successfully and never had an abort or any issues with baby rats or full grown.
I don't feed it to the babies obviously but I've given it to preggo moms and not had any issues at all.
I give them a "grain of rice" in size, wiped onto the front teeth of the rodent or given on a piece of bread for those rodents that bite(mine don't).
The way I was told of to rid rats of lice is revolution but it's not cheap.