This Jarom 1:8 phrase is used throughout the Book of Mormon and appears to borrow its language from the Abrahamic covenant in Gen 17:2 and elsewhere. It appears to be an apt description of the early Utah Saints, who saw themselves as modern heirs of the covenant. Mormon women welcomed many more children into their homes than their national counterparts, a phenomenon I attribute largely to polygamy.
As a disclaimer, I realize that this blog entry is not in good taste, especially from a feminist perspective. Some of the quantitative analysis that follows will no doubt feed into Mormon stereotypes that we like to keep our women barefoot and pregnant.
There has been much consternation over the years between the various branches of the restoration on whether Jacob 2 allows polygamy or not. Utah saints often interpreted verse 30, “For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things” as allowing an exception to the general prohibition. I am tempted to further this argument along by showing that polygamy facilitated the raising up of seed both qualitatively and quantitatively. My treatment is only a sideshow to the bigger debates about Jacob 2 and polygamy, which are covered elsewhere by FAIR volunteers (see link1 link2 link3).
The quantitative aspect has fallen out of favor since Stanley Ivins reported that “that 3,335 wives of polygamists bore 19,806 children, for an average of 5.9 per woman. An equal number of wives of monogamists, taken from the same general group, bore 26,780 for an average of eight. This suggests the possibility that the over-all production of children in Utah may have been less than it would have been without benefit of plurality of wives.” 
Kathryn Daynes  pointed out that Ivins’s sample is biased towards elite polygamists, those who were prominent citizens or who paid an inclusion fee to appear in Frank Esshom’s 1913 production Pioneers and Prominent Men of Utah. In terms of offspring, it easy to see that the number of offspring would be biased lower. Ivins notes “There is no conclusive evidence that any of Joseph Smith’s many plural wives bore children by him. Heber C. Kimball, with his forty-five wives, was the father of sixty-five children. John D. Lee, with only eighteen ‘true wives,’ fell one short of Kimball’s record, and Brigham Young fathered fifty-six children, approximately one for each wife.”
Smith and Kunz  used Esshom as well, but they created a sample so they could compare monogamous and polygamous fertility patterns. Since polygamous males, according to Ivins, married two wives (70.2%) and three wives (20.7%) the majority of the time, they limited their study to those situations. The following information considers only couples for which completed fertility rates could be determined. It gives the average number of children sorted by family type, before and after infertile wives removed.
One concurrent wife
wife 1: 7.82 7.99
wife 2: 4.79 6.83
Two concurrent wives
wife 1: 9.2 9.28
wife 2: 6.96 7.3
Three concurrent wives
wife 1: 7.24 7.5
wife 2: 6.81 7.15
wife 3: 6.06 6.76
Notice that bigamist families more than double the child count of monogamous families. The rates are no longer sustained when three concurrent wives are involved. The first wife of a polygamist was married at a slightly younger age than monogamous wives were, and that accounts for increase in number of offspring. Juggling three wives eventually countered the boost from the headstart. The finally tally was monogamous first wives 7.82 children and polygamous wives 7.46 children.
With those fertility rates, if we consider a society formed from Esshom’s set except eliminate the elite polygamists and the second wives married to serial monogamists, then about 56% of the women marry monogamously and the remaining 44% marry polygamously. The overall average would be 7.67 children per married woman. Kathryn Daynes reported a 99% marriage rate for Mormon women in Manti and compared that to a national rate as low as 92%. In other words, polygamy gets females married off more efficiently. So if, counterfactually, Mormons had only practiced monogamy and enjoyed the 7.82 fertility rate and yet had to take a hit of only 92% marriage efficiency, then the efficiency adjusted rate would be 7.14 children per women compared to 7.59 for the efficiently married polygamous society. Another influence of polygamy was to drive the bride’s age below the national average by a couple of years, which in turn resulted in an increase in children.
So if one accepts the reasoning above, than the introduction of polygamy into Mormon society caused more rapid increase than if Mormonism had stayed with monogamy. Compare the figures above with that of the national average for white females:
Another metric is the ratio of children age (0-4) to 1000 white women age (15-49) provided by Thorton .
Year US Utah
1850 613 846
1860 627 1097
1870 562 927
1880 537 848
1890 473 691
 Stanley S. Ivins, “Notes on Mormon Polygamy,” The Western Humanities Review, X (Summer, 1956); reprinted “exactly as it appeared” upon Ivins death in Utah Historical Quarterly 35/4 (Fall 1967). See Anonymous, “Tribute to Stanley S. Ivins,” Utah Historical Quarterly 35/4 (Fall 1967): 307–309. Hat tip to Gregory L. Smith for bringing this source to my attention.
 Kathryn M. Daynes, More Wives than One: Transformation of the Mormon Marriage System, 1840–1910, (Urbana, Illinois: University of Illinois Press, 2001) p. 103.
 James E. Smith and Phillip R. Kunz, “Polygyny and Fertility in Nineteenth-Century America,” Population Studies, Vol. 30, No. 3. (Nov., 1976), pp. 465-480.
 Arland Thornton, “Religion and Fertility: The Case of Mormonism,” Journal of Marriage and the Family, Vol. 41, No. 1. (Feb., 1979), pp. 131-142.