Sometimes fish act strangely. I did not answer before because I don't know how long they are pregnant for. I think it might be about 30 days. Plus they can give birth up to three times after mating once(at about 30 day intervals). That's what mine did. Not all the fry survive if left in the tank with the adults. I left mine in the tank and from one fish I had 24 survive, which is enough when you have to rehome some of them. When you do rehome be careful because some people feed them to larger fish ! That's ok if you don't mind, but I would.
If the fry breed together because they are brothers and sisters their fry can be deformed. I have this situation and as far as I know there are no fry yet. I am going to clean the gravel once a week and hopefully they will get sucked up while they are tiny.