We provide novel estimates of women's decision of when to have their first birth in terms of fertility timing (young vs. old) and season of birth (which quarter), for non-Hispanic white women aged 25-45 in the US in 2005-2013. The prevalence of good season (quarters 2 and 3) is very significantly related to mother's age, as well as to her education and marital status, while those who do not undergo assisted reproductive technology procedures to achieve their first birth exhibit a much higher prevalence of good season births. The frequency of good season is also higher and more strongly related to mother's age in states where cold weather is more severe, reinforcing the interpretation that season of birth is a choice outcome. Finally, we find important interactions between good season of birth and a woman's labour market choices. All of these facts point firmly to the fact that the season in which a baby is born is a choice variable made by the mother or family, potentially explaining a considerable portion of the quality difference observed in "good season" births.