Midrash for Exodus 34:19

Sanctify unto me all the firstborn (Exod. 13:2). This verse illustrates one of the thirteen rules by which the Torah is interpreted.¹²R. Ishmael’s thirteen hermeneutical rules; see also above, n. 10. A general statement requires a particular statement for its interpretation. Sanctify unto Me all the first born is the general statement which includes both male and female. Does this mean, however, that all firstborn, whether male or female, are to be considered as the firstborn? No, for Scripture adds: The firstling males that are born of thy herds and thy flock thou shalt sanctify (Deut. 15:19), thereby specifying males and not females. It may be possible to maintain on the basis of this verse that a fetus extracted by a Caesarean operation is also to be considered a firstborn. Hence the Scripture states: All that openeth the womb is Mine (Exod. 34:19). That is, a male that opens the womb (through natural birth). Both of men and beast. The firstborn of humans are likened to the firstborn of beasts. Just as the birth of a premature beast releases the one born after it from the law of the firstborn, so a firstborn human being, following a miscarriage, is released from the law of the firstborn. The Levites are not included in this commandment because they are not subject to the law of the firstborn whether of man or of beast.