Niddah 66 - Modern Law
Rav and Shmuel both said regarding the law of a newlywed virgin bride that her husband performs only one cohabitation, which is a mitzvah, and then abstains. Rav Chisda objected, "But it once happened that Rabbi Yehudah the Prince gave a newlywed virgin bride four nights, and these were spread out over twelve month!" The Talmud interjects - Rav Chisda should have asked from the ruling that we just learned , which makes the distinction of age! Rav Chisda preferred to base his question on a practical case, which is more authoritative, rather then on a theoretical ruling. But in any case, there is a contradiction!?
The answer is that Rav and Shmuel were talking about the law as it was established later, and the one that is active today. Since not everybody is familiar with all the laws of a virgin bride, and since there is also a concern that the husband's desire for his wife might overwhelm him, the Sages decided to give all virgin brides the law of the most stringent case of those discussed above . Thus, all virgin brides are given only the first mitzvah-cohabitation.
Another stringency that arose in later generations: Rabbi Zeira said, "Jewish women have accepted upon themselves the stringency that even if they see a drop of uterine blood the size of a mustard seed, they wait seven clean days because of it."
Art: Abbott Handerson Thayer - A Bride