Musar for Yevamot 156:1

והא אמר רב כהנא לא שנו אלא רובו אבל כולו חוצץ שאני עובר דהיינו רביתיה

surely [it may be retorted] R. Kahana stated: This applies only in respect of its greater part, but when the whole of it is effected a legal interposition is constituted!<span class="x" onmousemove="('comment',' Even if the person does not mind the interposition. In the case of the embryo, surely, all its body remains untouched by the water. Why, then, should the child be exempt from the immersion! ');">¹</span> — The case of the embryo is different since its position<span class="x" onmousemove="('comment',' In utero, during pregnancy. ');">²</span> is that of its natural growth.<span class="x" onmousemove="('comment',' The mother's body is inseparable from it and cannot, therefore, be regarded as an interposition. ');">³</span>

Orchot Tzadikim