From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger asks why marital relations with a pregnant wife are permitted given that the fetus (possibly his daughter) is considered part of the mother's body via ubar yerech imo, which would seemingly constitute a form of incestuous relations at least at a rabbinic level. He resolves the difficulty by distinguishing that intercourse with a female under three is already prohibited as hashchasas zera, so the Sages never separately enacted an ervah-based rabbinic prohibition for this category; consequently, in the case of normal marital relations with a pregnant wife, no such prohibition applies to the fetus.
Source: Shu"t Rabbi Akiva Eiger §172