The phrase “Thigh will rot” is a euphamism for a condition known as uterine prolapse or pelvic organ prolapse. Pelvic organ prolapse is a common condition in third world countries, in areas with poor nutrition, where women have many children and where women often perform strenuous manual labor. It was very common in the ancient world where women were expected to work hard and bear many sons. It is still a common occurence today. Women do kegel exercises when they are pregnant to prevent it.
Uterine prolapse (or Pelvic organ prolapse) occurs when the female pelvic organs fall from their normal position, into or through the vagina. Occurring in women of all ages, it is more common as women age, particularly in those who have delivered large babies or had exceedingly long pushing phases of labor. Smoking, obesity, connective tissue disorders, upper respiratory disorders‚ and repetitive strain injuries can all increase prolapse risk.
Jews would not mention the names of the genitalia but would use an evasive synonym such as thigh or foot instead.
*So the servant put his hand under the thigh of his master Abraham and swore to him in this undertaking. (Genesis 24:9)
Then Zippo’rah took a flint and cut off her son’s foreskin, and touched Moses’ feet with it, and said, “Surely you are a bridegroom of blood to me!” (Exodus 4:25)*
Abraham’s servant actually swore an oath with his hands on his master’s testicles but the writer used the evasive synonym thigh instead. Zipporah actually touched her son’s foreskin to Mose’s penis but the writer used the evasive synonym foot instead. Jewish writers used foot, thigh, leg, etc., as evasive synonyms in order to not have to write the actual names of the genitals.
As anyone could imagine, a woman with weak pelvic muscles and pelvic prolapse would be more likely to miscarry. Pelvic prolapse also sometimes results in a distended belly. The RSV translation gets it right.
And when he has made her drink the water, then, if she has defiled herself and has acted unfaithfully against her husband, the water that brings the curse shall enter into her and cause bitter pain, and her body shall swell, and her thigh shall fall away, and the woman shall become an execration among her people. But if the woman has not defiled herself and is clean, then she shall be free and shall conceive children. (Numbers 5:27-28)
This has nothing to do with abortion and the writer was simply using the word thigh as an evasive synonym for the word vagina. What the writer is describing is uterine prolapse.
The NIV is an extremely poor translation.