from Numbers in The New International Version of the Bible:
if you have gone astray while married to your husband and you have made yourself impure by having sexual relations with a man other than your husband”— 21 here the priest is to put the woman under this curse—“may the Lord cause you to become a curse among your people when he makes your womb miscarry and your abdomen swell. 22 May this water that brings a curse enter your body so that your abdomen swells or your womb miscarries.”
“‘Then the woman is to say, “Amen. So be it.”
23 “‘The priest is to write these curses on a scroll and then wash them off into the bitter water. 24 He shall make the woman drink the bitter water that brings a curse, and this water that brings a curse and causes bitter suffering will enter her. 25 The priest is to take from her hands the grain offering for jealousy, wave it before the Lord and bring it to the altar. 26 The priest is then to take a handful of the grain offering as a memorial offeringand burn it on the altar; after that, he is to have the woman drink the water. 27 If she has made herself impure and been unfaithful to her husband, this will be the result: When she is made to drink the water that brings a curse and causes bitter suffering, it will enter her, her abdomen will swell and her womb will miscarry, and she will become a curse. 28 If, however, the woman has not made herself impure, but is clean, she will be cleared of guilt and will be able to have children.
Some translations have something like "your thigh rots" instead of "your womb miscarries," or other words to the same effect as still other translations do. I don't know Hebrew and so cannot have an authoritative opinion about the correct translation, except to note that miscarriage or spontaneous abortion fits the theme of the passage much better than does a rotting thigh. One almost may as well say "I curse you: May your abdomen swell and a sausage grow on the end of your nose."
I don't really care what is correct here, in the sense that my own thoughts about what is right or wrong with abortion depend on it not at all. People who maintain that the Bible is absolutely authoritative are bound to cherry-pick when making nearly any point because so much is or seems to be contradictory. And because large sections of the Bible are vile and difficult to explain away and thus unsuitable for any discussion of personal ethics or social direction.
And, of course, if you believe dumbass stuff like this, it's easy to see how you can believe in other dumbass stuff like "Here, grab this red-hot iron bar and if you're innocent God will keep you from being burned."