A miscarriage is defined by the loss of the fetus before the 23rd week. Which this was.

Yes, loss meaning death of. The definition is such that it defines a spontaneous abortion right at the time before fetal viability. The loss of a fetus at this age automatically means death especially with the limited technology of the Berserk world. The definition is "the expulsion of a fetus from the womb before it is able to survive independently". The definition requires the fetus to die. No woman with a surviving preemie baby says they had a miscarriage, just a short pregnancy and there are plenty of people who survived during this window. Caska's pregnancy lasted only about a week but because of the magical influence of Femto the baby obviously was able to survive without additional gestation. Thus, not a miscarriage.

Granted Casca didn't lose her mind when she miscarriaged, but she lost it the same time her child was tainted by Femto

That was not the original parallel that I critiqued, so it really doesn't matter. To reiterate, Caska didn't lose her mind because of the termination of her pregnancy and correlation is not causation.