Commentary for Amos 1:13

they ripped up the pregnant woman of Gilead—They destroyed it and also ripped up their pregnant women so that no heir would remain who would contest the inheritance. And so does Scripture state: (Jer. 49:1) “Concerning the children of Ammon. So said the Lord: Has Israel no sons? Has he no heir? Why has Malcam taken possession of Gad?”

And here the decree that Hashem put on the Amonites is mentioned including the three sins of idolatry, immorality and murder that the Amonites did and the fourth sin that was regarding Israel. For all these, Gd will not overlook and will truthfully repay them according their deeds. And the fourth sin was ripping open the "haros" of Gilead in order to expand their boundary. And the word "haros" comes from the word "harim" mountains and it's the feminine form of the word. And this means that since they were neighbors with Gilead, they would break open the mountains that they were neighbors to to expand their borders and since we know "cursed is the one who encroaches on his fellow man's borders" and Jeremiah (49:1) said "to the sons of Amon thus said Hashem, 'has Israel no sons to inherit taht you inherited their land of Gad and dwelled in their cities" . Jeremiah mentions Gad because it is capitol of Gilead nad has access to crossing the Jordan. And rashi provides another explanation for the word "haros" and says it refers to pregnant women. The Amonites were so unmerciful in their quest to destroy every remnant of Israel that is why the prophet says "has Israel no children to inherit"