This is the first paper that documents that school entrance cut-off date affects the timing of births. Many countries require children to reach a certain age by a specified date in the calendar year in order to start primary school. There is a clear tradeoff for parents to time a birth after the school entrance cut-off date; delaying births after the cut-off date cost parents additional year of child care through sending their children to school at a relatively older age, while births just after cut-off date may benefit children by being in the oldest among the school cohort, which is shown to provide the children with academic advantage (relative age effects). Using the universe of birth during 1974–2010 in Japan with single school entrance cut-off date of April 2, I reach three findings. First, I find that more than 1,800 births are shifted roughly a week before the cut-off date to a week following the cut-off date. My finding suggests that relative age effects may dominate at least in Japanese setting. Second, I find that non-working mothers are more likely to delay births, which is consistent with lower opportunity cost of non-working mothers. However, interestingly among those working, mothers with low skilled jobs are more likely to shift births before the school entrance cut-off dates. This result implies that financial burden of child cares may force mothers to time delivers earlier to avoid the additional one year of child care cost. Also I find that delay of births is much more drastic for male births than female births, which may be one form of son-preference. Finally, I examine the health outcomes of births. While birth after the cut-off date was slightly heavier, I do not find any change in infant mortality, which is plausible since hospitals anticipate the surge in the births. This study may have implications for growing literature that assumes births around the school entrance cut-off dates are random.