One of the women about whom the first Mishna (2a) taught that there is no mitzva of yibum is the case of eishet ahiv she-lo hayah be-olamo – when there is a brother who was not born until after the woman became a widow. In such a case, the severe prohibition of a person marrying his brother’s wife remains in effect, and the woman does not have to wait until the newborn child is old enough to perform yibum or halitza; she can get married immediately.
This halakha is mentioned as one of 15 cases in the first Mishna in the massekhet; the first Mishna of the second perek- which begins on our daf– deals with it in some detail. The Gemara suggests that the source for this halakha is the passage that opens the rules of yibum (see 25:5), beginning with the condition “when brothers live together,” which implies that this is a law that applies only when the brothers were alive at the same time.
Tosafot ask whether the rule of derakheha darkei no’am (see 3:17) should be applied in this case. Derakheha darkei no’am – the idea that the commandments of the Torah are pleasant and considerate – is used in many circumstances in the Talmud to clarify that a given interpretation cannot be considered because it would make fulfilling the Torah an unpleasant experience. In our case, it should be fairly obvious that forcing the widow to wait and see whether a brother will be born to her late husband would contradict the idea of derakheha darkei no’am.
The answer suggested by Tosafot is that the pasuk brought as a source by our Gemara includes even a case when the late husband’s mother was pregnant, a situation that would not have been covered by derakheha darkei no’am.
In truth, the Talmud Yerushalmi does make use of the idea of derakheha darkei no’am, albeit without using that specific language, arguing that were the yevama (the widow) to be obligated to wait for the possibility of a new brother, surely the Torah would have mentioned the role played by her in-laws within these laws, since the possibility of their having more children impacts on the status of the yevama.