The Effects of Meteorological Conditions on the Circadian Rhythm of Births.

Objective: The aim of this study is to evaluate the influences exerted by temperature on the circadian rhythm of births.

Methods: In the past births occurred mainly between midnight and dawn, while today births tend to be less frequent during the night hours. Today, almost all births are hospitalized and, therefore, they may adapt to the organizational requirements of the health care structures and staff. This piece of research regards births in Udine, a city in north-eastern Italy, recorded at the beginning of the 19th century. The data on births come from the French civil register. Weather data come from very detailed daily collection with three measurements per day. From the statistical point of view, we apply methods developed for circular data. In order to highlight relationships between time of birth and explanatory variables, we estimate MANOVA (multivariate analysis of variance) models and perform a statistical test for comparison between groups.

Results: The test against the homogeneity of the hour-of-birth distributions across the 4 seasons is significant (p < 0.01). One-to-one comparison of the distributions is performed via Watson's two-sample test on data for each couple of seasons: the Summer-Autumn comparison is the only non-significant test of homogeneity. Moreover, we consider daily temperatures in the bivariate model in several different ways and transformations. The daily temperature effect is more significant if considered as the difference between the evening temperature of the day before the birth and the mean daily temperature of the same day, over the nine-year period 1807-1815. Finally, based on this division of the births, Watson's two-sample homogeneity test for the distribution of the hour of birth is significant (p < 0.05).

Conclusions: We found that the circadian rhythm of births is influenced by temperature, with an anticipation of the time of birth on warmer days. To our knowledge, our results are the first evidence of the effects of daily temperature on the time of birth.