Why is there no concern for Havchanah in the case of a Yefas To'ar according to the opinions that you need not wait 90 or 120 days (R' Shimon Ben Elazar and Ravina)?
David
The Kesef Mishneh (Hilchos Melachim 8:6) writes that the reason why the first Bi'ah is permitted immediately (even though it is necessary to wait three months when she converts) is because at that stage she is a Nochriyah so the child is anyway not considered the son of the father.
Dovid Bloom
I should point out that, on the contrary, Rebbi Shimon ben Elazar, at the beginning of 48b, states that one does indeed require 90 days, and Ravina asks that, if so, why should one not require 120 days?
The aforementioned Kesef Mishneh writes that the reason for these requirements is because of Havchanah.
Kol Tuv,
Dovid Bloom