Praise be to Allah.
Shaykh al-Islam Ibn Taymiyah was asked about a Christian woman whose husband was Muslim; she died when there was a seven month old foetus in her womb. Should she be buried with the Muslims or with the Christians?
The answer was that she should not be buried in the Muslim graveyard, or in the Christian graveyard, because in this case we have a Muslim and a kaafir together, and a kaafir cannot be buried with the Muslims or a Muslim with the kaafirs. Rather she should be buried on her own, with her back towards the Qiblah, because the child is facing her back, so if she is buried in this manner the face of the Muslim child will be towards the Qiblah. The child is a Muslim because his father is Muslim, even though his mother was a kaafir, according to scholarly consensus.