One concern in contemporary society – that women are marrying later, and that at a later age fertility rates are lower and it is more difficult to conceive – was a matter of concern for the daughters of Zelopheḥad, as well.
As we have learned, the daughters of Zelopheḥad appealed to Moshe that they deserved their father’s share in the land of Israel, given that he died with no sons. Our Gemara quotes a baraita that credits these women with being intelligent, well spoken and righteous. According to the Rashbam, their righteousness was clear from the fact that they listened to Moshe’s advice and married within their tribe so that their father’s inheritance would not be transferred to another tribe. Furthermore, it appears that they were very selective, waiting to marry an appropriate husband.
The Gemara notes, however, that this led all five daughters to marry at a late age – Rabbi Eliezer ben Ya’akov taught that the youngest was 40 when she got married. Tosafot suggest that this is obvious from dating the story in the desert. If Zelopheḥad died immediately after the sin of the spies, and his daughters only married in the 40th year when the land was being divided up, clearly they were at least 40 years old.
This leads the Gemara to ask how they could have had children at that advanced age, given Rav Ḥisda’s teaching that a woman who marries before the age of 20 can conceive until she is 60. If she marries after 20, she can conceive until she is 40. If she marries after she is 40, she can no longer have children. The Gemara responds that they were deserving of a miracle due to their righteousness, and they succeeded in conceiving.
One possible explanation for Rav Ḥisda’s teaching is based on women’s physiology. Every woman is born with a limited number of ova. When a woman has her period, at least one ovum is released. During pregnancy, the hormones in a woman’s body do not allow for menstruation, a condition that often continues while a woman nurses. Thus, a woman who marries young, gives birth and nurses her children, has a much larger number of ova remaining in her later years, while a woman who marries late and has been menstruating throughout her childbearing years, has fewer possibilities for conceiving when she marries.