Reference for Yevamot 16:1
הָאִשָּׁה שֶׁהָלַךְ בַּעְלָהּ וְצָרָתָהּ לִמְדִינַת הַיָּם, וּבָאוּ וְאָמְרוּ לָהּ מֵת בַּעְלֵךְ, לֹא תִנָּשֵׂא וְלֹא תִתְיַבֵּם, עַד שֶׁתֵּדַע שֶׁמָּא מְעֻבֶּרֶת הִיא צָרָתָהּ. הָיְתָה לָהּ חָמוֹת, אֵינָהּ חוֹשֶׁשֶׁת. יָצְתָה מְלֵאָה חוֹשֶׁשֶׁת. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, אֵינָהּ חוֹשֶׁשֶׁת:
If a woman's husband and her tzarah went abroad, and they came and said to her: Your husband died, she may not remarry [(since her husband had left without children)], and she may not be taken in yibum until she knows that her tzarah had not been pregnant, [lest her tzarah had given birth. And if one would ask, let her be given chalitzah and then let her marry "to the marketplace" whatever the case — the gemara answers: A chalutzah is unfit for the priesthood, and if she were given chalitzah and then it were found that her tzarah had borne a "surviving" child and that the chalitzah were meaningless, she would have to be proclaimed as kasher for the priesthood, not having been rendered unfit by that chalitzah. And if one had been present at the chalitzah, but not at the "rescinding" proclamation, and thereafter saw her married to a Cohein, he would say that a chalutzah is permitted to a Cohein.] If she had a mother-in-law [abroad], she need not fear [that she had been "given" a yavam. Even though above we do fear lest her tzarah might have given birth, that is because whatever she bore, male or female, it would release her from yibum; but, as to the mother-in-law, where even if she did bear, the first is yibum-linked only if she bore a son, we entertain the possibility that she might have miscarried, or (even if we assume that she had not miscarried,) that she might have borne a female.] If she (the mother-in-law) went out "full" (i.e., pregnant), she does fear [that she might have been given a yavam.] R. Yehoshua says: She does not fear. [The halachah is not in accordance with R. Yehoshua.]