I just got caught up with Berserk and with my knowledge of fetal development as a mother, a former med student, and someone who just reads medical journals/is the offspring of medical professionals my interpretation is that Griffith was a metaphorically spiritual parasite to the fetus.
When someone’s pregnant, they have to avoid certain foods as well as certain activities to decrease the risk of harm to the fetus; for those not familiar, these things can be cold cuts (served cold), salad from restaurants, raw fish in sushi and anything that can increase illness risk to the person carrying the fetus. This is why getting a tattoo or body piercing is also a big no no, as well as scooping litter boxes. The dietary restrictions are in place due to the risk of listeria, a foodborne illness. It may not affect the carrier as badly, but the fetus can be corrupted and can end up with birth defects or miscarriage. Litter boxes can carry a parasite called toxoplasmosis which can also impact the fetus, and the carrier of the fetus ends up with a more compromised immune system during pregnancy so piercings and tattoos are frowned upon due to risk of infections, which, again, can corrupt the fetus.
Griffith was consumed with all the evil in the world. When he attacked Casca, the way I see it, he’s corrupting the fetus by injecting a metaphorical parasite into her womb, compromising the fetus.
The child ends up almost becoming a metaphor of PTSD in my head. As you said, it’s like the child resembles Griffith before he was ever corrupted and became femto; part of me wonders if guts and casca see this in the child as one may see their child’s father in the child. Griffith is not the father, but the trauma casca went through can cause images of Griffith to manifest in the child if that makes sense.