Sample collection
Fecal samples were collected from the diapers immediately after defecation. The first samples (meconium) were collected in the hospital by the study or NICU nurse within three days postpartum (i.e. 0 months sample). The timing of sample collection did not differ among the study groups. The later (1, 6 and 12 months) samples were collected by parents at home, immediately frozen to −20°C, and delivered to the study clinic within 24 hours. In the study clinic, the samples were frozen and stored at −80°C until analysis.