From the UN Declaration of Human Rights:
Article 15
1. Everyone has the right to a nationality.
2. No one shall be arbitrarily deprived of his nationality nor denied the right to change his nationality.
Many countries, even if they aren't signatories to the UNDHR, include a similar principle in their laws.
A question for those of you who argue that a fetus is a person: how should this principle be applied to fetuses?
Normally, birthright citizenship gets conferred based on, well, a person's place of birth. Before that, though... is a fetus a stateless person? If so, is that a violation of the fetus's rights?
Should we recognize a nationality of the fetus? If so, how should that be determined? Based on the nationality of the parents? The place where the fetus was conceived?
In cases where the nationality of the fetus doesn't match how we determine the nationality of the child, is this an arbitrary deprivation of the child's nationality?