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. 2015 Nov 18;15:300. doi: 10.1186/s12884-015-0738-2

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© Sun et al. 2015

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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Fig. 4 — Funnel plots for the meta-analyses assessing the associations between PM_2.5 exposure (per 10 μg/m³ increment) during pregnancy and preterm birth. a: In studies that assessed PM_2.5 exposure during the entire pregnancy. b: In retrospective studies. c: In prospective studies. d: In studies conducted in the USA