Rapid changes in American families have reshaped inequalities in child well-being. This paper examines the unequal consequences of family structures for infant health, focusing on birthweight. Existing studies mainly address the average association between marriage (versus singlehood) and birthweight. I extend the literature by 1) explicitly considering cohabitation and 2) exploring the heterogeneous associations based on mother's likelihood of union formation at conception. Pooling nationally representative data from the National Survey of Family Growth 2011-17, I analyze a sample of recent births (N = 4,376) born to mothers aged between 20 and 49 years. Propensity score methods are used to address selections. Results show that 1) compared to single mothers, married mothers reap birthweight benefits, while cohabiting mothers do not; 2) married mothers with a higher likelihood to marry at conception (i.e., more advantaged) reap even larger birthweight benefits than their low-likelihood counterparts (i.e., less advantaged). Overall, the findings reveal important and nuanced roles of family structure in the reproduction of intergenerational inequality through infant health.

美国家庭的快速变化重塑了儿童福祉的不平等。本文探讨了家庭结构对婴儿健康的不平等影响，重点关注出生体重。现有研究主要探讨婚姻（与单身）和出生体重之间的平均关联。我通过 1）明确考虑同居和 2）探索基于母亲在受孕时形成结合的可能性的异质关联来扩展文献。我汇集了 2011-17 年全国家庭成长调查的全国代表性数据，分析了年龄在 20 至 49 岁之间的母亲所生的近期出生样本 (N = 4,376)。倾向评分方法用于解决选择问题。结果表明：1）与单亲母亲相比，已婚母亲获得出生体重优势，而同居母亲则没有； 2) 受孕时结婚的可能性较高（即，更有优势）的已婚母亲比低可能性（即，优势较小）的已婚母亲获得更大的出生体重优势。总体而言，研究结果揭示了家庭结构在通过婴儿健康再现代际不平等方面发挥着重要而微妙的作用。