As we have learned (see above, daf 7) since we do not anticipate that a pregnant woman or a nursing mother will menstruate, it suffices for them to reckon their period of uncleanness from the time of their discovering menstrual blood; we do not require them to assume that everything with which they came into contact in the previous 24 hours (or since the last time that she examined herself for purity) will be rendered unclean, as is the case with other women.
At what point will a woman be deemed to be pregnant or nursing?
According to the Tanna Kamma , as soon as it is apparent that she is pregnant or when she actually begins breastfeeding, this rule will apply. Rabbi Yose argues that there must be a change in her menstruation cycle. Only after she has missed her period for three months in a row will we view her as pregnant or nursing.
When discussing Rabbi Yose’s opinion, the Gemara on today’s daf suggests that the two cases can be combined. Thus, according to Rabbi Yose, if a nursing woman becomes pregnant the laws of ritual purity will be applied to her and we will not need her to miss another three menstrual cycles before we recognize the pregnancy.
Tosafot asks how it could be possible that a nursing woman could become pregnant, given the fact that according to the Gemara (later on in Massekhet Nidda, daf 38b) a woman only becomes pregnant close to her immersion in the mikvah (others say close to her ovulation), which only takes place if she is menstruating! Tosafot answers that the Gemara did not mean to say it is possible to become pregnant only at those times, rather that those were the optimal times for pregnancy to take place. For this reason, pregnancy can occur in a nursing mother without her ever menstruating between the two.