I imagine that in the Death Parade universe this happens often enough to have a specific arbiter rule about it. Any details of such a rule haven't been revealed yet however.
It is the woman and the man's souls that are being judged in that trial, so I presume the baby either
- Has a separate case for review, possibly not present in the woman's body in body/soul during the woman's trial
- Is not eligible for trial due to not having enough life experience.
The Rebirth/Void concept that Death Parade uses is based on Buddhist concepts, which means that an opinion-based answer could be highly debatable as it would be based on theological concepts such as the possibility of humans being born malicious, or becoming malicious through life's influence.
References if you want to decide by yourself:
Reincarnation: The Process of Rebirth