pca Ahrens 2014.

Methods	Retrospective cohort study. This study was performed by retrospective analysis of data within the Birth Defects Study, an ongoing case‐control study investigating the occurrence of neonatal malformations conducted by the Slone Epidemiology Center at Boston University. It includes hospitals serving the areas surrounding Philadelphia and San Diego, Rhode Island, southern New Hampshire, and parts of New York State and Massachusetts. Occurrence of preterm delivery and low birth weight were compared between non‐malformed (controls) infants born from vaccinated and unvaccinated mothers.
Participants	For the study seasons included in this analysis (from 2006–07 to 2009‐10), mothers of live‐born, non‐malformed infants were identified; 1619 were included in the study.
Interventions	Immunisation with trivalent inactivated vaccine during pregnancy. Within 6 months after delivery, a study nurse conducted a phone interview asking for information about immunisation (and other issues). Women reporting influenza vaccination during pregnancy were asked to provide a release to allow study staff to obtain their vaccination records, but only 60% of the women complied with this request. Reports of seasonal trivalent influenza vaccination were categorised according to the timing of receipt: any time during pregnancy (last menstruation period to day before delivery), first trimester (last menstruation period through 14 weeks), second trimester (greater than 15 through 28 weeks), and third trimester (greater than 29 weeks to day before delivery). Women who reported vaccination with pandemic H1N1 vaccine were excluded from the analysis. 334 women were immunised for all seasons considered in the study.
Outcomes	Gestational age at delivery and birth weight were obtained by self report from the mother during the interview. Gestational age was determined by calculating the difference between the last menstruation period and the day of delivery. If the self reported last menstruation period date differed by more than 7 days from the last menstruation period date calculated from the reported ultrasound‐determined due date, then the latter last menstruation period date was used to calculate gestational age. If the self reported last menstruation period date differed by 7 days or less from the last menstruation period calculated from the due date, we chose to use the self reported last menstruation period date because it was a date familiar to the mother and raised less confusion during the course of the interview. Small for gestational age, defined as a weight < 10th percentile for gestational age, considering the sex‐specific distribution of birth weights of infants born in the US in 1999–2000 Preterm delivery, defined as live birth before 37 weeks' gestation ILI: for the last season in study (2009‐10), having had ILI symptoms was also ascertained during the interview
Notes	Funding source ‐ government At the time of manuscript preparation, Katherine Ahrens was a pre‐doctoral Boston University Reproductive, Perinatal and Pediatric Epidemiology trainee supported by the National Institutes of Health (Grant T32 HD052458). Data collection for this project has been funded by the Biomedical Advanced Research and Development Authority, Office of the Assistant Secretary for Preparedness and Response, Department of Health and Human Services (Contract No. HHSO100201000038C); the Agency for Healthcare Research and Quality (Grant 1R18HS018463‐01); and the National Institutes of Health (Grants 1R01 HD059861 and 2 R01 HD46595). Drs Louik and Mitchell and Mr Kerr receive research support from Novartis Vaccines and Diagnostics (NVD) for an unrelated study of a meningitis vaccine. Dr Mitchell serves as a member of an advisory committee for a pregnancy registry for a multiple sclerosis agent conducted by Biogen Idec and as an unpaid consultant to NVD on matters unrelated to influenza vaccines. Dr Werler has provided consultation for Amgen, Bristol‐Meyers Squibb, and Abbott regarding their pregnancy registries for rheumatoid arthritis drugs. These companies do not manufacture influenza vaccines. Dr Ahrens has no conflicts to disclose.
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS ‐ selection exposed cohort All outcomes	Unclear risk	Description is insufficient. Participants for this study are simply selected from control population of the Birth Defects Study.
PCS/RCS ‐ selection non‐exposed cohort All outcomes	Low risk	Selected from the same population as the exposed cohort
PCS/RCS ‐ comparability All outcomes	Unclear risk	Not clear whether all possible confounding factors were considered
PCS/RCS ‐ assessment of outcome All outcomes	Unclear risk	Interview
Summary assessment	Unclear risk	Unclear risk of bias