Rashi on Genesis
בעד כל רחם GOD HAD CLOSED UP EVERY רחם — means had closed up every opening (of the body)
Sforno on Genesis
כי עצר עצר ה' בעד כל רחם, to kill their (unborn) offspring if Avimelech would not display remorse and penitence. G’d had warned him specifically that not only his own life was at risk (verse 7)
Or HaChaim on Genesis
כי עצור עצר ה׳ בעד כל רחם. For G'd had blocked all body orifices. This was so that people should not be able to say that Sarah had become pregnant by Avimelech. Avimelech was impotent during that period. The Torah also wanted to demonstrate that Abraham's prayer was effective to make the formerly barren fertile again. We also learn from this that if someone prays for someone else to be healed in an area that he himself needs healing, his own needs will be attended to by G'd first. This is why the next verse tells us about Sarah having become pregnant. While it is true that G'd had already promised this previously, if a certain sequence in scripture is not applicable to the subject under immediate discussion, it may be applied to another subject as will be discussed shortly.