The Mishna (59b) teaches that one of the obligations that a wife has to her husband is to nurse his newborn child. This is a responsibility that exists even after her husband dies. According to the baraita brought by our Gemara, should her husband pass away while she is nursing, the widow cannot marry for a year and a half (Rabbi Yehuda) or two years (Rabbi Meir) lest she become pregnant and lose her ability to produce milk.
During pregnancy, a woman’s body secretes a variety of hormones. While prolactin stimulates the production of milk, other hormones, like progesterone and estrogen reduce its production. (Until recently these hormones were given to women who chose not to breastfeed their babies.) Oftentimes – although not in every case – the presence of these hormones in a pregnant woman minimizes and eventually brings to an end the production of milk. It should also be noted that as the embryo develops, it begins to monopolize whatever the mother’s body produces, so that if her nutrition is not rich enough to support both nursing and pregnancy, her supply of milk will stop.
Recognizing the limits of a woman’s body to support more than a single nursing child, the Gemara quotes a baraita that forbids a woman from nursing another child – even her own – if she was hired to nurse another child. Furthermore, if the amount of food agreed upon was not enough to sustain a nursing woman, she must eat more. The Meiri and the Rivan understand this to mean that if her agreement with the baby’s mother does not include enough food, she must supplement on her own. Others understand this to refer to the food that she is receiving based on her agreement with her husband. If it is not enough to support a nursing woman, she must add more to the agreed-upon amount.