title: Control of confounding requires specification of risk periods.
authors.forename: C
authors.initials: C
authors.lastname: Thijs
authors.forename: P
authors.initials: P
authors.lastname: Leffers
authors.forename: P
authors.initials: P
authors.lastname: Knipschild
mesh_terms: D016022:Case-Control Studies; D002769:Cholelithiasis / Q000453:epidemiology / Q000209:etiology; D015986:Confounding Factors, Epidemiologic; D005260:Female; D006801:Humans; D015994:Incidence; D016017:Odds Ratio; D011247:Pregnancy; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects; D013997:Time Factors