Rabbi Yochanan taught a strict rule in Yevamot 34b: a widow who waits ten years before remarrying will have no children with her new husband. The ten-year gap, the sages believed, shuts the womb.
Rav Nachman softened the verdict. "Only if she did not intend to marry during those years," he said. "If all the while she was hoping for a husband — her body kept itself ready."
There was a woman who proved the point. Rava married Rav Chisda's daughter ten years after her first husband's death, and she bore him children. One day he teased her: "The Rabbis are doubtful about you." She answered without flinching: "I had always set my heart on you." Her will had kept the door open.
Another woman once came to Rav Yoseph. "I waited ten years and still bore children," she said, as if disproving the teaching. Rav Yoseph did not soften. "Daughter," he said, "do not bring the words of the sages into discredit. You are mistaken, not they." And she broke down and confessed — she had transgressed during the waiting years.
The body keeps the faith the heart keeps first.