John Gill 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
Or "the womb", and therefore to be sanctified, and set apart for his use: this also was declared, and the law concerning it given, at the time of their coming out of Egypt, and here repeated, (See Gill on Exodus 13:2), (See Gill on Exodus 13:12).