Numbers 3:12

12. Openeth the matrix Or womb. “This expression is generally employed in cases in which a common term is required to designate the firstborn of both man and beast, (Exodus 13:2; Exodus 13:12-15; Exodus 34:19-20; Numbers 8:16-17; also Numbers 18:15; Ezekiel 20:26;) but even then, wherever the two are distinguished, the term פשׂר , peter, is applied to the firstborn of animals, and בכור to the firstborn sons of men. On the other hand, where only firstborn sons are referred to, as in Deuteronomy 21:15-17, we look in vain for the expression peter rechem ’openeth the womb.’ Again, the Old Testament, like modern law, recognises only firstborn sons, and does not apply the term firstborn to daughters at all.” Keil and Delitzsch. The law of inheritance omits the daughter in designating the firstborn. In case of polygamy there was only one firstborn.