What is the shivisah of a child born on shabbos or yom tov?
Mendy kaplowitz
The child definitely goes after the Shvisah of the mother (see Mishnah Berurah 414:8, who gives the reasoning that a young child is like the mother because "k'Gufa Damya" -- "he is like her body" and "Ee Efshar Lo b'Lo Imo." Even the Elyah Rabah quoted in Mishnah Berurah 414:7 would agree that it goes after the mother, not the father, in this case.)
All the best,
Yaakov Montrose
It would seem to me that is not talking about where the child was not extant at bein hashmoshis, so a shvisah cant begin on shabbos or yom tov.
Being that we hold "Ubar Yerech Imo" -- "A fetus is like the thigh of his mother" (see SHACH Y.D. 79:8) then the child should be considered to be in existence Erev Shabbos, and we can certainly apply the Mishnah Berurah's reason of "k'Gufa Damya."
Kol Tuv,
Yaakov Montrose