There's a danger in using translations such as the NIV, which have been deliberately edited to create contradictions such as this one.

The vast majority of English translations seem to speak of the woman undergoing some sort of disfigurement if she were guilty. Her thigh would waste away, and her abdomen would swell. We checked over a dozen Bible versions, and only two versions specifically mention a miscarriage in place of the thigh wasting away. The NRSV speaks of the “womb discharging,” and the 2011 update of the NIV states that her womb would “miscarry”—the 1984 edition matches the other versions mentioned above. The NCV offers a different understanding from the rest by stating that God would make her “body unable to give birth to another baby.”

https://answersingenesis.org/sanctity-of-life/numbers-5-and-abortion/