Charles Ellicott Commentary Exodus 34:19

"All that openeth the womb is mine; and all thy cattle that is male, the firstlings of cow and sheep." — Exodus 34:19 (ASV)

All that openeth the matrix is mine; compare Exodus 13:12, where the sanctification of the firstborn and the law of redemption had already been declared. For the exact mode of redemption sanctioned, see Numbers 18:15-16.