Numbers 5:11-31, describes an unusual procedure, known as the jealousy offering, a husband could use to determine if his wife had been unfaithful to him. Essentially, the husband and wife would come to the priest, the priest would then create a concoction of unpleasant ingredients, and then the wife would have to drink the concoction. If the wife was guilty of adultery, she would get sick and her belly would swell. If the wife was innocent, God would protect her from the effects of the concoction. There was nothing magical about the concoction. It was entirely a matter of God using the result to demonstrate whether a woman was innocent or guilty. So, in summary, Numbers 5:11-31 affirms the truth of Numbers 32:23, "Be sure your sin will find you out."
Some propose that Numbers 5:11-31 refers to God causing an abortion. The 2011 edition of the NIV mistakenly states that the drink will cause miscarriage in Numbers 5:21-22, 27. However, this is not what the passage is talking about. Pregnancy is nowhere mentioned, or even hinted at, in the text. The only thing that even sounds like pregnancy is the guilty wife’s stomach becoming bloated, but even in that instance, it has nothing to do with pregnancy. Further, the passage does not say that drinking the concoction would cause an abortion/miscarriage. While drinking a poisonous mixture of ingredients could very well cause a miscarriage, that is not what this text is speaking of.
If a wife was found guilty, the punishment was death (Leviticus 20:10). If the wife was found innocent, she would be “cleared of guilt” and “able to have children” (Numbers 5:28). So, again, Numbers 5:11-31 does not refer to abortion in any sense. Rather, it is describing a method that God allowed to be used to determine if a wife had committed adultery against her husband.