Does single pregnancy adversely affect infant health? This is a challenging research question because there is selection in coresidence during pregnancy. We exploit quasi-natural variation in single pregnancy from the moment of conception to birth, arising from the reform of the marriageable age laws in Korea. The Korean birth certificate data are unique. They provide the information about the coresidence status in addition to the legal marital status. The coresidence start date allows us to identify the intensity of coresidence during pregnancy. We find that coresidence or legal marriage during pregnancy does not significantly affect birth outcomes.

中文翻译：

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单胎怀孕会影响年轻母亲的分娩结果吗？

单胎妊娠对婴儿健康有不利影响吗？这是一个具有挑战性的研究问题，因为在怀孕期间可以选择同居。我们利用韩国适婚年龄法改革引起的从受孕到出生的单胎妊娠准自然变异。韩国出生证明数据是独一无二的。除了合法婚姻状况外，它们还提供有关同居身份的信息。同住开始日期使我们能够确定怀孕期间同住的强度。我们发现怀孕期间的同居或合法婚姻不会显着影响分娩结果。