According to the early sage Rabbi Judah, a woman’s pregnancy lasts exactly nine months and the length does not depend on a specific number of days, only on months. That is, in his opinion it doesn’t matter if the months are full (30 days) or deficient (29 days) — the pregnancy will last exactly nine Jewish months and the number of days during the pregnancy will change based on the sages’ rulings on new months and their length. The scholars asked: If so, why did one of the scholars [for it is not customary for later scholars to contradict the opinion of earlier sages], Samuel, argue that the length of a woman’s pregnancy is exactly 271 days from conception, in contradiction with the opinion of the early sage who stated that pregnancy could be longer or shorter? Though the scholar Samuel argues that conception can happen up to three days after sex, so the baby could be born 272 or 273 days after sex, how can a later scholar disagree with an earlier sage? Answer: The scholar Samuel had other early sages upon whom he based his ideas, the early pious ones who thought that a woman’s pregnancy lasts exactly 271 days. These pious men would therefore have sex with their wives only on Tuesday, Wednesday, Thursday, and Friday nights, so the women would not give birth on the Sabbath, causing a violation of the holy day. Had they had sex with their wives on Saturday, Sunday, or Monday nights their wives may have given birth on the Sabbath. This is the calculation — if they had sex on Monday night and conception happened that same night, we count 38 weeks (266 days), getting to a Monday night, and then add the rest of the required days to equal 271 days (5 more days) and the woman will give birth on the Sabbath, leading to Sabbath violation. The scholars asked: How did the early pious ones know the precise length of a woman’s pregnancy? Answer: They learned it from the verse ” So Boaz took Ruth and she became his wife; and when he went in to her, the Lord gave her conception [herayon], and she bore a son” (Ruth 4:13). They converted the word herayon into its numerical value (gematria), which is 271, the days of a woman’s pregnancy. One of the scholars, Mar Zutra, added that though the early pious men thought a woman’s pregnancy to last exactly 271 days, no longer and no shorter, this is if she’s entered her ninth month. If she gives birth in the seventh month, her pregnancy need not last exactly seven months (210 days) but can be longer or shorter. From whence this distinction? From the verse “So it came to pass after seasons of days that Hannah conceived and bore a son” (I Samuel 1:20). Seasons means that her pregnancy lasted two seasons, being six months, and days means her pregnancy lasted an extra two days [the sages interpreted times written as plural in the Scriptures to mean only two, so seasons would mean two seasons and days would mean two days]. Therefore Hannah’s pregnancy lasted 182 days and not 210.
(Babylonian Talmud, Tractate Niddah 38a-b)