Defining the Firstborn for Inheritance and Redemption

The Rambam separates paternity from the opening of the womb and establishes when emergence constitutes legal birth.

Irrevocable Vows Before Temple Service

The Gemara discusses a priest who married a woman forbidden to the priesthood. Before performing the sacrificial service, the bet din (rabbinic court) compels him to take an oath or vow to divorce her, ensuring he will not serve while planning to remain with her. To prevent him from seeking halakhic absolution, the court administers the vow על דעת רבים (based on the consent of the public).

The Rambam codifies this procedure, ruling that a vow made on the public's consent cannot be nullified, guaranteeing he will fulfill his obligation to divorce her. In the laws of oaths, he clarifies that while an ordinary vow taken in public may be dissolved, one dependent on public consent can never be released except exclusively for the purpose of a mitzva. The Kessef Mishneh notes that this follows the ruling of Ameimar, who permits absolution solely for a mitzva, as in the Gemara's case of the meticulous schoolteacher,.

אֵינוֹ עוֹבֵד עַד שֶׁיַּדִּירוּהוּ בֵּית דִּין עַל דַּעַת רַבִּים כְּדֵי שֶׁלֹּא יִהְיֶה לוֹ הֲפָרָה

He may not serve until he is compelled by the court to take a vow dependent on the discretion of other people so that it cannot be nullified. (Mishneh Torah, Admission into the Sanctuary 6:9)

Inheritance Versus Redemption of the Womb

The Mishnah establishes that the double portion of inheritance and the redemption of the firstborn to a kohen are fundamentally separate categories. Inheritance depends entirely upon the father, whereas redemption depends solely upon the mother's womb.

The Rambam rules that a בכור לנחלה (firstborn for inheritance) is determined strictly as the father's first offspring, based on the verse 'the beginning of his strength'. The maternal lineage is disregarded; even if the mother has borne many children, the father's biological firstborn receives a double portion. Conversely, redemption depends solely on being a פטר רחם (first issue of the womb). Any miscarriage that imparts the ritual impurity of childbirth exempts subsequent sons from redemption, whereas a discharge that does not impart birth impurity leaves the next child obligated.

The Head of a Miscarriage

In the Gemara, the sage Shmuel maintained that the emergence of the head does not exempt subsequent births in the case of a נפל (non-viable fetus). The Gemara decisively refutes Shmuel, demonstrating from tractate Niddah that the emergence of the forehead establishes birth even for a miscarriage.

The Rambam codifies this conclusion against Shmuel: if an eight-month fetus extends its head while alive and retreats, or a nine-month stillborn extends its head, the twin born after it is not a firstborn for redemption. The emergence of the forehead alone completes the womb-opening. For animal fetuses, the Rambam similarly rules that extending the head renders the offspring born, requiring its own slaughter.

Regarding inheritance, the Rambam adds that if a full-term fetus brought forth the majority of its head alive, a subsequent son is not a firstborn. The Maggid Mishneh asserts that this reflects the refutation of Shmuel. The Kessef Mishneh and Lechem Mishneh analyze this remark, noting that Shmuel disputed only the redemption of five sela'im, whereas all agree that a live full-term head precludes inheritance rights for a subsequent child,.

זֶה שֶׁיָּלְדָה אֵינוֹ פֶּטֶר רֶחֶם שֶׁהֲרֵי נִפְטַר בְּרֹאשׁוֹ שֶׁל רִאשׁוֹן וּמִשֶּׁתֵּצֵא פַּדַּחְתּוֹ פּוֹטֵר הַבָּא אַחֲרָיו

The one she bore is not the first issue of the womb, for it was exempted by the head of the first, and as soon as its forehead emerged, it exempted the one who comes after it. (Mishneh Torah, First Fruits and other Gifts to Priests Outside the Sanctuary 11:15)

Forehead Recognition in Practice

The Gemara records a dispute between Rabbi Shimon ben Lakish and Rabbi Yochanan regarding whether the emergence of a forehead alone establishes birth for inheritance. The Torah states יכיר ('he shall recognize'), which Rabbi Shimon ben Lakish interprets as requiring recognition of the entire face. Rabbi Yochanan rules that the forehead alone suffices.

The Kiryat Sefer explains that the Rambam adopts the rule that the forehead constitutes valid recognition for inheritance. The Gemara contrasts this with the laws of freeing an עגונה (chained wife), where witnesses must identify the forehead, nose, and full facial countenance to confirm a husband's death. In practice, if a child's forehead emerged into the world while the father was alive, he is the legal firstborn for the double portion, and any child whose forehead emerged from the mother's womb fulfills the womb-opening, exempting all subsequent siblings from the redemption payment to the kohen,.

Inheritance requires only that the child was the father's first live birth recognized by the emergence of the forehead, while redemption depends strictly on the physical opening of the mother's womb.

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