Mothers explicitly target spring and summer when planning to give birth, according to research by Climent Quintana-Domeque and colleagues, to be presented at the Royal Economic Society''s annual conference in Brighton in March 2016.
Their study examines tens of millions of first births in the United States. It finds that far few babies are born between November and March, while far more are born between June and September. By contrast, countries in the southern hemisphere have more births in December to March and fewer in May to August, their summer and winter respectively.
The study also finds that younger women (especially those aged 28-31), women not using artificial reproductive technologies and women working in occupations with long holidays during summer are considerably more likely to have their births in spring or summer, when newborn health is highest.
Meanwhile, in US states where winters are harshest young women are almost 8% more likely to give birth in summer than in winter. This is not the case for older women, even if they are first-time mums. The authors comment:
''For the first time, our research documents that season of birth is actually a planned decision: mothers have a clear desire to target the ''good'' seasons of birth of spring and summer – and often manage to do so.''
The season of the year in which babies are born has considerable impacts on their health at birth, and also their adult achievements. In the United States, winter months are associated with lower birth weight, education and earnings, while spring and summer are found to be ''good'' seasons.
The central findings from this study suggest that mothers explicitly target these ''good'' seasons when planning to give birth. Younger women (especially those aged 28-31), women not using artificial reproductive technologies and women working in occupations with long holidays during summer are considerably more likely to have their births in spring or summer, when newborn health is highest. These results are found to be particularly striking in areas where winters are harshest.
By examining tens of millions of first births from US birth certificates, a number of new striking stylised facts are documented. First, in the United States (the northern hemisphere) there is a clear trough in the number of births occurring between November and March, with a corresponding peak in the warmer months of June and September.
But when examining a southern hemisphere country, these results are inverted. In Chile, births are preferred in December to March, and avoided in the winter months of May to August.
This pattern is not simply a biological relationship. It is found to hold in a much more consistent fashion for those first-time mothers who have more flexibility in timing their births. Women who have less flexibility in targeting birth months exactly – for example because they are undergoing artificial reproductive technologies (ART) – are found to have no ''good season'' advantage.
If we consider the effect of harsh winters, young women are found to respond very clearly to bad weather. In US states where winters are the most extreme, younger women are nearly 8% more likely to give birth in summer than in winter (54% summer versus 46% winter).
Among older women, no such gradient is found. Older first-time mums (for example those aged 40 and above), are equally likely to give birth in summer or winter, and there is no gradient found, even in the case of the coldest winters. Similarly, women who engage in ARTs like in vitro fertilisation are found to have no power over the timing of their birth, with births spread equally throughout the year, with the exception of a downward spike in December conceptions due to shorter clinic hours.
Finally, labour market flexibility has important implications: for certain occupations, lulls in contact hours during summer months provide an additional incentive to time births during these periods. In US Census data, women working in educational sectors (teachers, educators and librarians) are observed overwhelmingly to target, and give birth, in the spring and summer (53.6% of all births occur in summer, versus only 46.7% in winter).
For the first time, this research documents that season of birth is actually a planned decision: mothers have a clear desire to target these ''good'' seasons of birth, and often manage to do so. But where mothers are subject to restrictions in birth timing, whether these are personal, such as the age at which they begin trying to conceive, or professional, such as lack of labour market flexibility, their ability to time births optimally to take advantage of ideal conditions is considerably reduced.
About the Research: The results from this note are documented in the article ''Choosing Season of Birth: The Role of Biological and Economic Constraints'', which will be presented by Dr. Climent Quintana-Domeque at the Royal Economic Society Conference, 2016.
About the Authors: Dr. Climent Quintana-Domeque is Associate Professor at the Department of Economics at the University of Oxford. Dr. Damian Clarke is Associate Professor at the Faculty of Economics at the University of Santiago Chile. Dr. Sonia Oreffice is Professor in the School of Economics at the University of Surrey.