Mice not mating, are they already pregnant?

I had 4 females and 4 males together for 3 days then separated each female into her own cage. A week later all 4 had built nests (they were said to have possibly been pregnant when I got them) and were starting to show signs of pregnancy. Everything about them showed that they were going to have babies but they did not or at least I never saw them. I came to the conclusion that they were never pregnant and I was just being hopeful. I mixed them back in with the males and one female and one male mated all night so I put her in her own cage. However after four days no other mice are mating, do you think they are already pregnant and just don't want to mate?
Also, I have 2 rats together, one of them had baby's and then two days later the other one had babies butt they are dead but she did not eat them. Do you think they were born dead or do you think she killed them? She does not appear to have eaten them...

Wow! Sorry for the dilemma. I am breeding mice and rats on a small scale. I have had success with 1 male breeding with several females. I don't think keeping similar numbers of males and females together is the best strategy. With the males it will be a dominant factor and can result in fighting , injuries and maybe even death housing males together when females are present. They can remain housed as bachelor groups.

I actually started out with pairs of one male and one female. This time I have two males and two females. So far I've only seen one couple getting busy and with as many occurrences I am assuming she's pregnant

First 4 males with 4 females is not the best idea, males will spend more times fighting each other and dammaging each other than they will doing the deed.

There are a few factors also are they prime for breeding, did you buy them as adults, if you did they might be past that prime.

With both rats and mice you want to buy young weaned animals and raise them up to breed them.

As for rats it can be stillborn, it can be lactation problem from the mother however usually in that case females will cull them and eat them.



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give the females a little time off before reintroducing one male into the cage; leave him in the cage until the females are like ping pong balls then remove the male. The females might be stressed with all the males and have eaten their litters. If they don't go for you soon id get new breeders to replace them.