Because of Sarah

Notice that at the end of Genesis 20 the reason that Yahweh had closed up the wombs of the women in Abimelech's house is translated "because of Sarah". The Hebrew actually says "on the word of Sarah" which seems to suggest that she prayed for God to protect/judge her abusers, etc. That's kind of cool. But also of interest is the fact that 'davar' (the word for 'word') has the same consonants as the word for pestilence or plague 'dever'. So there's a bit of a pun going on there. This fits because this chapter is an Exodus in miniature, with God's people being oppressed by a wicked king, God's people lying to protect themselves, and in the end being delivered from the king's hand and going out with great possessions (another example is Gen. 12). But in this instance, the 'plague' God sends is Sarah. And, as my wife pointed out, Sarah is barren. So God sends barrenness on the women of Abimelech's house. But of course after God has opened the wombs of the Philistines, He opens Sarah's womb too.