Reference for Ketubot 204:13

ומאי פסקא איהי דהואי בשעת קנין מהני לה קנין בנות דלא הוו בשעת קנין לא מהני להו קנין

How do we know this? Since she was in existence at the time of the kinyan, the kinyan is effective for her; the other daughters who were not in existence at the time of kinyan, the kinyan does not work for them.

Jastrow