Reference for Bava Batra 284:3
מר בריה דרב יוסף משמיה דרבא אמר לומר שממעט בחלק בכורה ודוקא בן יום אחד אבל עובר לא מאי טעמא (דברים כא, טו) וילדו לו אמר רחמנא
teaches<span class="x" onmousemove="('comment',' Lit.. 'to say'. ');"><sup>7</sup></span> that he<span class="x" onmousemove="('comment',' A child who is one day old. ');"><sup>8</sup></span> causes a diminution in the portion of the birthright.<span class="x" onmousemove="('comment',' I.e., if there are, e.g.. two brothers exclusive of the child, the estate is divided not into three portions (two for the two ordinary portions of the two brothers and one for the birthright, but into four portions. Each brother, including the child, receives one such portion and the firstborn receives the additional fourth portion as his birthright. The firstborn thus receives, as the portion of his birthright, a quarter of the estate, and not, (as would have been the case if the child were excluded). a third. ');"><sup>9</sup></span>