TY - JOUR AU - Strand, Linn Beate AU - Barnett, Adrian G. AU - Tong, Shilu PY - 2011 DA - 2011/04/18 TI - Methodological challenges when estimating the effects of season and seasonal exposures on birth outcomes JO - BMC Medical Research Methodology SP - 49 VL - 11 IS - 1 AB - Many previous studies have found seasonal patterns in birth outcomes, but with little agreement about which season poses the highest risk. Some of the heterogeneity between studies may be explained by a previously unknown bias. The bias occurs in retrospective cohorts which include all births occurring within a fixed start and end date, which means shorter pregnancies are missed at the start of the study, and longer pregnancies are missed at the end. Our objective was to show the potential size of this bias and how to avoid it. SN - 1471-2288 UR - https://doi.org/10.1186/1471-2288-11-49 DO - 10.1186/1471-2288-11-49 ID - Strand2011 ER -