The Elfe cohort comprises more than 18.000 children (whose parents consented to their inclusion), all selected purely on the basis of their date and place of birth.

In each of the 344 selected maternity units, midwives "targeted" babies born during four specific periods representing each of the four seasons in 2011: 1-4 April, 27 June - 4 July, 27 September - 4 October, and 28 November - 5 December. All children born during one of Elfe’s four recruitment periods in selected maternity units in metropolitan France were eligible to take part in the study. These 344 maternity units were selected by drawing lots, taking account of their size in order to weight the birth sample.

To ensure representativeness, foreign families could also take part in the study, providing the mother could read in French, Arabic, Turkish or English. These are the languages most frequently spoken by the foreign mothers who give birth in France and it was in these languages that the information letters and consent forms available at the maternity units were printed.

Nonetheless, several specific situations were deemed to be incompatible with the study. Multiple births of more than two children, underage parents or parents who were not capable of giving informed consent were excluded. Similarly, children born before 33 weeks of amenorrhoea (i.e., before the eighth month of pregnancy) were not included in the cohort.
Some parents were invited to take part in Epipage 2, which had the same recruitment periods as the Elfe cohort. The aim of this study is to learn more about the outcomes of very premature infants. Elfe is cooperating extremely closely with the leaders of this project.

 Number of families taking part in the survey region by region