The Code of Canon Law states, "Every person not yet baptized and only such a person is capable of baptism" (CIC 864). Therefore, a non-baptized person who is not yet born is theoretically capable of baptism.
The problem with such a baptism is how to do it. Given that water is the proper matter for baptism, is there a safe way apply it in utero? While the current (1983) Code does not prohibit the baptism of children in the womb, the 1917 Code stated, "No one should be baptized in the mother’s womb so long as there is a hope that he can be baptized correctly outside of it" (1917 CIC 746). This law is no longer in force, but the practical difficulty of safely baptizing the unborn remains.