The Torah discusses the laws of firstborn children with regard to two different areas of halakha: redeeming a firstborn male child (see 13:2 and 18:15-16) and inheritance laws, where the Torah grants a double portion to the firstborn male child (see 21:15-17). Although it is possible – and even likely – that the same child will both have to be redeemed and will receive a double portion, these two situations have very different rules determining who fits into the category of “firstborn.” Regarding redeeming a firstborn child, the criterion is peter reḥem – the child that opens its mother’s womb. In contrast, the child that is considered the firstborn with regard to inheritance laws is the one who is reishit ono – “the first-fruit of his strength.” Thus, a father can have a number of firstborn sons that must be redeemed, inasmuch as every woman who he marries may give birth to a firstborn son that “opens her womb.” He can only have a single firstborn son that will get a double portion of inheritance, however, and that child may be born from a woman who has already given birth previously in a different marriage.
The first Mishnayot in this perek examine different situations and circumstances that will affect the status of a firstborn child with regard to each of these laws. For example, cases where the mother miscarries or gives birth to a stillborn child will likely impact on the status of the firstborn live child with regard to the laws of redeeming the firstborn, since the birth of that child may not be considered the one that “opens her womb.” It will not, however, impact on the firstborn with regard to inheritance laws.