Dubno Maggid: Shemot III
The Midrash teaches us that Yocheved, Moshe’s mother, was miraculously restored at a very old age to being a young woman. The Ibn Ezra, among others asks why the Torah would not explicitly describe such a great miracle as it did regarding Sarah?
Imagine two beggars meeting upon returning from the same town. One goes on and on describing their generosity, and the other is shocked for all he received was a penny or two from the same people. How is it possible that two people had such drastically different stories to tell about the same village?
After a few moments the two men figured out that the successful beggar visited the village on Purim, when everyone gives away gifts to the poor. The unsuccessful beggar came on a regular weekday when everyone was busy running around trying to make a living.
So too, when Abraham was alive, miracles were rare, so the Torah goes to great lengths to describe Sara’s change from a 89 year old woman into a young girl. However, everyone in Egypt was experiencing miracles, as the Sages teach, women were giving birth to six healthy children at a time. During such a period, the miracle of Yocheved was not so special, and the Torah does not mention it.