Lee Bean and Geraldine Mineau report LDS plural wives had fewer children than non-plural-wives.
Lee Bean and Geraldine Mineau, "The Polygyny–Fertility Hypothesis: A Re-evaluation," Population Studies 40, no. 1 (June 4, 2010): 67-81
Our data clearly indicate an inverse relationship between wife-order and completed fertility, and these differences persist in spite of the introduction of a variety of statistical controls.