Yevamot, Chapter Sixteen, Mishnah One
This mishnah deals with a woman whose husband died abroad and she is not sure whether or not she is obligated to have yibbum with his brother.
1) A woman whose husband and rival wife went to a country beyond the sea, and to whom people came and said, your husband is dead, must neither marry nor contract yibbum until she has ascertained whether her rival wife is pregnant.
2) If she had a mother-in-law she need not be concerned [she had another son].
a) But if [the mother-in-law] departed while pregnant she must be concerned [that another son was born.
1) Rabbi Joshua says: she need not be concerned.
Section one: In this case, the husband and a rival wife went abroad, and at that time the husband had no children. Although the wife who remained behind is certain that her husband is dead, she does not know whether she is liable for yibbum, because she doesnt know if her rival wife had children while abroad. Therefore she cannot remarry.
Section two: If the husband traveled abroad and at that time he had no brothers, his wife need not be concerned lest her mother-in-law gave birth to a child before her husband died. Perhaps this is because it is uncommon for a woman to give birth to another child after her older child is already married. Furthermore, even if the mother-in-law gave birth, only a male child will make the wife liable for yibbum. This distinguishes this case from the case in section where a child of other sex will exempt the wife from yibbum, and therefore we are concerned lest she be exempt.
If, however, the mother-in-law was pregnant when she traveled, the daughter must be concerned, lest she gave birth to a male child. The wife must wait until she is certain that her mother-in-law did not have a boy.
Rabbi Judah holds that in this case as well, the wife need not be concerned and may remarry immediately. This is because there are two doubts. The mother-in-law may have miscarried (as older women often do), and even if she gave birth to a viable child, the child might be female. In cases of two doubts, the halakhah is generally more lenient.