2.2 Study population and design
We conducted a retrospective cohort study by analyzing data from children born between 2009 and 2016 from the TMCHD. A total of 1459093 labor events were identified initially. After excluding mothers with foreign nationality (n =88837), stillbirths or miscarriages (n = 14639), and newborns with missing identification numbers (n = 67274), there were 1288343 mother-child pairs (Figure S1). The exposure of antibiotics during pregnancy, as well as the dose and timing, was identified by using Anatomical Therapeutic Chemical Classification (ATC) code J01. The index date was set as the birth date. The trimester in which antibiotic exposure occurred was defined as follows: first, 1-91 days; second, 92-189 days; third, ≥190 days. The outcome variable was defined as outpatient visits at least 3 times or one hospitalization with a diagnosis of atopic dermatitis (ICD-9-CM: 691 or ICD-10-CM: L20) after the index date. Maternal comorbidities were defined as comorbidities occurring two years before the index date and at least 3 outpatient visits or one hospitalization, whereas gestational infections, gestational diabetes, and preeclampsia were defined as at least one outpatient visit or hospitalization. The exposure and non-exposure group were followed up until the onset of atopic dermatitis, death, or 31 December 2019, whichever occurred first.