Reference for Niddah 53:19
ואמרי לה בית שערים מעשה ונשתהה ולד אחד אחר חבירו ג' חדשים והרי הם יושבים לפנינו בבית המדרש
what can be said [in explanation of the incident]?<span class="x" onmousemove="('comment',' Apparently nothing whatever. If the first was born in the seventh month (even if on the last day) and the second in the ninth month the interval would not be one of thirty-three days but one of no less than two months. If they were both born in the seventh month the interval would inevitably be less than thirty-three days (since a Hebrew month never contains more than thirty days). If again, one was born in the seventh and the other in the eighth month the latter could not be viable, whereas the incident which speaks of a welad ('child') and not of nefel ('abortion') seems to refer to two viable children. ');"><sup>16</sup></span>