Yevamot 36 - Chalitzah with a pregnant widow
If one's brother dies and leaves a pregnant widow, chalitzah or yibum should not be required - since they apply only when a man dies childless. If one still goes ahead and performs a chalitzah, it is meaningless at this time.
However, if later she miscarries and it turns out that the brother did die childless after all - then now the chalitzah would be required, and the question arises: does the previously performed chalitzah suffice? Rabbi Yochanan gives a general rule: a chalitzah of a pregnant woman is actually valid, because now it transpires that she did need one. Likewise, a yibum done with a pregnant woman also counts for the future. Why? Should Elijah the Prophet (who knows the future) come and reveal to us that she will miscarry, then the chalitzah would be valid - and therefore it is valid even now. Alternatively, says Rabbi Yochanan, the proof is from the rule we had learned before, which states that " the marriage can continue ." This means that the yibum with the pregnant woman is valid!
However, Resh Lakish disagrees on all counts and declares the chalitzah or yibum invalid. As far as Elijah and his knowledge of the future - Resh Lakish answers that right now at least we do not know the future, and should judge accordingly, not using the logic of retroactively changing the meaning of current events based on the future outcomes. And as far as the rule of "the marriage can continue" - which seems to imply that the original yibum was valid - Resh Lakish will answer that it was not. Rather, a new yibum is required, and then the marriage can continue.
Art: The Widow by Pavel Andreevich Fedotov