Why did Joseph Smith marry women who were already pregnant?

Joseph Smith likely received his revelation about plural marriage around 1831 while he was translating the Bible. The first plural marriage that historians have any fragmentary evidence of is Smith’s marriage to Fanny Alger in the mid-1830s. After this marriage ended, it seems that Smith stopped the practice until the 1840s. 

After Mormons moved to Nauvoo, Illinois, Smith began to practice plural marriage again. Multiple people recorded that Smith was warned by an angel at least three times that he needed to recommit to the practice of plural marriage, or he would be destroyed. It was under this heavenly threat that Smith reentered the practice of polygamy in Nauvoo.

The majority of Joseph Smith’s first plural wives in Nauvoo were already married (and pregnant) women. Smith might have chosen these women so that he did not have to have sexual relationships with these women or to protect Emma (Smith’s first wife). Joseph was actively preaching that you should not have sex while you are pregnant. It seems as though Joseph was marrying women who were far along in their pregnancy to show he was avoiding the appearance of impropriety. Later, Smith would marry single women. Some of the plural marriages that Smith entered into were dynastic sealings, a type of familial binding, like Smith’s marriages to Helen Mar Kimball and Rhoda Richards.

Joseph Smith married the majority of his wives in Nauvoo, Illinois between 1839 and 1843. Joseph married his last plural wife, Fanny Young, in November 1843.