The Elfe cohort (Longitudinal Study from Childhood) will analyze perinatal exposures to environmental, nutritional, and infectious pollutants and assess their impact on the health of 20,000 children born in 2011 and followed through adulthood. The collection of biological samples in the maternity ward is a crucial step in assessing these exposures. A pilot study was conducted in October 2007 involving 500 births, showing an 80% success rate for biological sample collection in the maternity ward from umbilical cord blood, breast milk, urine, and the mother’s hair. The samples were collected by midwives and transported twice daily at +4°C to the two EFS centers, where they were aliquoted and stored at -80°C. This pilot study validated the procedures for collecting and biobanking samples and provided logistical, organizational, and scientific insights to scale the study to the national level. These lessons pertain to the selection of equipment, the refinement of protocols, the optimization of aliquoting and transport procedures; the establishment of regional coordination; the compensation of midwives; and the optimization of the sampling plan. The study also made it possible to select relevant matrices both quantitatively (volumes collected, number of subjects) and qualitatively (initial assays) for umbilical cord blood, maternal urine, hair, and milk collected in the maternity ward; to exclude others (milk collected at home); and to propose other avenues (maternal venous blood, meconium, placental tissue), and in this sense, serves as a rich source of insights for future biomonitoring studies. (R.A.)