Reference for Yevamot 237:4
לא תנשא ולא תתייבם וכו': בשלמא יבומי לא דדלמא מיעברא וקפגעה באשת אח דאורייתא אלא לא תנשא אמאי הלך אחר רוב נשים ורוב נשים מתעברות ויולדות
'HER RIVAL'?<span class="x" onmousemove="('comment',' Emphasis on HER. ');"><sup>10</sup></span> — It is this that we are told: {The possibility of a birth in respect] of that rival<span class="x" onmousemove="('comment',' Who went with her husband to a country beyond the sea. ');"><sup>11</sup></span> need be apprehended; in respect of another rival, however, it need not be apprehended.<span class="x" onmousemove="('comment',' If witnesses testified that the known rival (v. supra n. 11) was not pregnant there is no need to apprehend the possibility of a marriage with another wife who may have given birth to a child. ');"><sup>12</sup></span>