What a difference a day makes: quantifying the effects of birth timing manipulation on infant health.

Scheduling births for non-medical reasons has become an increasingly common practice in the United States and around the world. We exploit a natural experiment created by child tax benefits, which rewards births that occur just before the new year, to better understand the full costs of elective c-sections and inductions. Using data on all births in the U.S. from 1990 to 2000, we first confirm that expectant parents respond to the financial incentives by electing to give birth in December rather than January. We find that most of the manipulation comes from changes in the timing of c-sections. Small birth timing changes, even at full-term, lead to lower birthweight, a lower Apgar score, and an increase in the likelihood of being low birthweight.