Niddah 10: Nursing, Menstruation and Pregnancy

It is understood that a woman will not begin menstruating again until approximately two years after childbirth assuming that she is nursing.  The rabbis discuss how we would know that a woman would be pregnant or nursing.  The Tanna Kamma says that when one can see that she is pregnant and from the beginning of nursing, the rules regarding her status with ritual purity.  Rabbi Yosei says that she must have missed her period for three months in a row to be assessed as pregnant or nursing.

The Gemara considers Rabbi Yosei's opinions to be mutually exclusive.  If a nursing woman becomes pregnant, the laws of ritual purity then apply and she does not need to miss another three menstrual cycles before recognizing the pregnancy.  

Unlike any woman, the rabbis wonder whether it is possible for a nursing woman to become pregnant.  Later in this Massekhet (daf 38b) we will learn that a woman only becomes pregnant close to her immersion in the mikvah (which would also be close to her time of ovulation), which only happens when she is menstruating.  Tosafot suggests that it is understood that women can become pregnant at different times but that these were the optimal times for conception.  Thus a woman can become pregnant while nursing without having had a period between the two pregnancies.