According to the study, if births were not planned, the proportion of births that would take place on each of the seven days of the week should follow a discreet uniform probability distribution. When distribution is uniform, the expected proportion of births for each day of the week is about 14.28%.

Nevertheless, as the study shows, in the last decades there has been an increase in the births which take place in the central days of the typical working week. The days with a largest number of births are Wednesdays and, especially, Tuesdays. In the 1970-1979 period, the births on Tuesdays were the 14.71% of all the births of the week whereas in the 2000-2009 are the 16.98%. Since the 70s there is an important reduction in the number of births that take place on Sundays (14.15 in the 1960-1969 period compared with 10.56% in the 2000-2010 period) that extends, with greater intensity during the last two decades, to Saturdays.

By using the data of the register of inhabitants of the Valencian Community, the researchers empirically show that the current organization of the work times in the health sector is having an impact on the weekly distribution of births since doctors are imposing their hegemonic position. There is evidence of a change in the birth distribution, by days of the week, from the middle of the 20th century to date.

The management of the health resources, the risks inherent to pregnancy (Ronda et al., 2009), the fatigue of the mother and, especially, the adaptation of the work environment have favoured, together with other social aspects, a more active control of the birth moment by the health workers. The exact time of birth not only corresponds to a random biological process but its programming, even its inducement are increasingly frequent actions.

The hospital and interventionist birth represents a cultural pattern of assistance, in which the humanisation disappears in the pursuit of the total annulation of risk and to adapt it to the organization of the work time of our society, which is increasingly concentrated from Monday to Friday.

In fact, the hypothesis of this research is that the birth date corresponds to internal organisation reasons of the hospitals.

Only in specific dates (Christmas, Holy Week and summer holidays) the general pattern of a largest number of births on Tuesdays and Wednesdays is not repeated. The way in which the Spanish society organises its work and leisure times, especially in the health field, is significantly modifying the weekly distribution of births in Spain. It is proved that the weekly distribution of births has changed from a coherent uniformity in 1970 to a weekly cycle structure, which is more in line with the typical work week, on which social organisation aspects have an impact.

‘The ways in which we are socially organised (together with the doctor-patient power relationship) have unsafe impacts on biological variables, the results of this work also have implications in the actuarial and demographic fields’, says the paper. ‘It could also introduce unexpected perturbations in samples constructed for research in social sciences and clinical trials. The reason is that in the experiment control the date of birth is among the most commonly used variables in the systematic allocation of individuals to groups’.

The article also explains that the caesarean has progressively increased in the Spanish health system. For instance, in 2001 there was a caesarean rate of 22.45% while in 2005 it was about 25.20%. This means an incensement of 12.3% in the caesarean proportion on the total of births (Ministerio de Sanidad y Política Social, 2009).

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