Background: There has been a rapid spread of non-marital childbearing in the Czech Republic during the last two decades. At the same time, the low birth weight rates of children born to married and unmarried mothers have converged.

Objective: The goal is to explain the diminishing gap in low birth weight. Two explanations are assessed: the changing selection of unmarried mothers from disadvantaged socio-demographic groups, and increasing social support for unmarried mothers.

Methods: Data from birth register are analysed. Marital status (married vs. unmarried) disparities in low birth weight are modelled using logistic regression. Further analyses are then performed with a detailed measurement of partnership status. This detailed variable is partially missing and is thus supplemented with multiple imputation.

Results: The main explanation for the narrowing gap between the outcomes of children born to married and unmarried mothers is the increasing social support for unmarried mothers. Unmarried motherhood has become less detrimental to a child’s birth weight net of maternal demographic characteristics. The decline in selection from disadvantaged socio-demographic groups has also contributed to the convergence. However, the convergence of birth weight trends towards marital children seems to refer mostly to children of partnered mothers, with children of single mothers lagging behind.

Conclusions: The positive trends in the health of non-marital children are interpreted as being the result of the increasing institutionalisation of parenthood in non-marital unions. However, this does not apply to unpartnered motherhood, which continues to represent a health disadvantage.