Hi,
The mother’s Holy Communion was between the Lord and her. While the child was near the Blesses Sacrament in the mother’s body, the child was not receiving Holy communion into its own body. (Even in the womb, the child lives its’ own separate life with its’ own blood system and type.) Further, the child was not receiving sanctifying grace from the Eucharist because it was not yet Baptized. It is the sanctifying grace of the sacrament of Baptism that makes it possible for us to receive all the other sacraments by incorporating us into the life of Christ.
Fr. Vincent Serpa, O.P.