Contraceptive use and an ideal number of children are among the factors that significantly influence the fertility of a country. Sub-Saharan Africa in general and Nigeria in particular are noted for polygynous marriages. The relationship between polygyny and fertility is well documented in literature. However, the influence of polygyny on fertility in Nigeria is yet to receive adequate academic attention. This paper sets out to re-examine the polygyny-fertility hypothesis. Data were sourced from the 2013 Nigeria Demographic and Health Survey, which was a nationally representative survey of women of reproductive age (15-49). The survey interviewed women on fertility, family planning, marriage history, and maternal and child health. The 26,403 married women that provided responses on questions concerning marital unions and fertility behavior constitute the sample for the study. The findings show that although marriage type has no significant effect on the fertility of married women, rank among wives in polygynous unions significantly influence fertility with the first wife being more prolific than subsequent wives. Population policies aimed at reducing high fertility should factor in the role of polygyny and competition among wives for higher number of children.