Walk through Amos, verse by verse, with the great Bible Commentaries of Matthew Henry (1662 - 1714) & Adam Clarke (1760 - 1832)
1:13
Thus saith the LORD; For three transgressions of the children of Ammon, and for four, I will not turn away the punishment thereof; because they have ripped up the women with child of Gilead, that they might enlarge their border: KJV
{The Lord saith these things, On three great trespasses of the sons of Ammon, and on four, I shall not convert him, for he carved the women with child of Gilead, for to alarge his term.} Wycliffes Bible
Because they have ripped up - This refers to some barbarous transaction well known in the time of this prophet, but of which we have no distinct mention in the sacred historians.....a piece of cruelty the very mention of which strikes a horror upon one’s mind; one would think it is not possible that any of the human race should be so inhuman. Hazael was guilty of it, 2 Kings 8:12.