Rashi on Genesis 20:18/רש"י על בראשית כ׳:י״ח

1בעד כל רחם GOD HAD CLOSED UP EVERY רחם — means had closed up every opening (of the body)

2'על דבר שרה וגו BECAUSE OF (literally, by the word of) SARAH — at Sarah’s bidding (Genesis Rabbah 52:13).