Mollies ?

[Jaws22]

Do female mollies often die after giving birth?

[linuxrulesusa]

Have you been adding any aquarium salt?  IIRC mollies prefer more brackish water though they will survive in fresh.  That plus the stress of the fry could have caused it, possibly?

I've had guppies die after birthing, though maybe 1 out of 5?   1 out of 10?  It's not 50/50 but it does happen.

[Thriftyfisher]

In my experience if they do it is usually after their first drop.  It doesn't mean that you have done anything wrong unless it happens with all of your females.  As mentioned, salt can help as can harder water (add crushed coral or Texas holey rock) and temps in the upper 70's.