{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,13]],"date-time":"2026-08-13T14:39:12Z","timestamp":1786631952987,"version":"build-2736575974"},"reference-count":42,"publisher":"American Chemical Society (ACS)","issue":"11","license":[{"start":{"date-parts":[[2017,12,29]],"date-time":"2017-12-29T00:00:00Z","timestamp":1514505600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"published-print":{"date-parts":[[2018,11,1]]},"abstract":"<jats:title>Abstract<\/jats:title>\n                  <jats:sec>\n                    <jats:title>Background:<\/jats:title>\n                    <jats:p>Previous studies have investigated the associations between perfluoroalkyl acids (PFAAs) in women and time to pregnancy (TTP). Inconsistent results may be explained by differences in conditioning on parity.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Objectives:<\/jats:title>\n                    <jats:p>We used causal directed acyclic graphs to illustrate potential confounding related to previous pregnancies and exposure measurement error due to differences in the interpregnancy interval in pregnancy-based studies that include parous women. We exemplified the potential importance of these issues using data from the Danish National Birth Cohort.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods:<\/jats:title>\n                    <jats:p>We used discrete time survival models to estimate associations between maternal plasma PFAAs in early pregnancy and TTP in 638 nulliparous and 613 parous women.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results:<\/jats:title>\n                    <jats:p>PFAA quartiles were not associated with the TTP in nulliparous women. In parous women, higher PFAA quartiles were associated with longer TTP. The strongest associations were estimated for perfluorohexane sulfonate and perfluorooctane sulfonate. PFAA concentrations were higher in women with longer interpregnancy intervals. Accounting for the interpregnancy interval attenuated the estimated associations.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusions:<\/jats:title>\n                    <jats:p>Associations between PFAAs and TTP in parous women may be biased by confounders related to previous pregnancies and exposure measurement error. To avoid these biases, studies that include parous women may need to condition on a) common causes of PFAAs and the TTP in the index pregnancy, b) previous births (a descendant of a collider), c) PFAA levels or common causes of PFAA levels and the TTP in the previous pregnancy (to alleviate collider stratification bias caused by conditioning on previous births), and d) the interpregnancy interval (in pregnancy-based studies). Alternatives would be to restrict studies to nulliparous women or to use toxicokinetic modeling to correct exposure estimates in parous women. These recommendations may be extended to studies of other chemicals with similar toxicokinetic properties. https:\/\/doi.org\/10.1289\/EHP1493<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1289\/ehp1493","type":"journal-article","created":{"date-parts":[[2018,11,12]],"date-time":"2018-11-12T07:21:01Z","timestamp":1542007261000},"source":"Crossref","is-referenced-by-count":9,"title":["Conditioning on Parity in Studies of Perfluoroalkyl Acids and Time to Pregnancy: An Example from the Danish National Birth Cohort"],"prefix":"10.1021","volume":"126","author":[{"given":"Cathrine","family":"Carlsen Bach","sequence":"first","affiliation":[{"name":"Viborg Regional Hospital Department of Pediatrics and Adolescent Medicine, , Viborg,","place":["Denmark"]},{"name":"Aarhus University Hospital Department of Pediatrics and Adolescent Medicine, , Aarhus,","place":["Denmark"]},{"name":"Aarhus University Hospital Perinatal Epidemiology Research Unit, , Aarhus,","place":["Denmark"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Niels","family":"Bjerreg\u00e5rd Matthiesen","sequence":"additional","affiliation":[{"name":"Aarhus University Hospital Department of Pediatrics and Adolescent Medicine, , Aarhus,","place":["Denmark"]},{"name":"Aarhus University Hospital Perinatal Epidemiology Research Unit, , Aarhus,","place":["Denmark"]},{"name":"Randers Regional Hospital Department of Pediatrics and Adolescent Medicine, , Randers,","place":["Denmark"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"J\u00f8rn","family":"Olsen","sequence":"additional","affiliation":[{"name":"Aarhus University Section for Epidemiology, Department of Public Health, , Aarhus,","place":["Denmark"]},{"name":"University of California, Los Angeles Department of Epidemiology, Fielding School of Public Health, , Los Angeles, California,","place":["USA"]},{"name":"Aarhus University Hospital Department of Clinical Epidemiology, , Aarhus,","place":["Denmark"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tine","family":"Brink Henriksen","sequence":"additional","affiliation":[{"name":"Aarhus University Hospital Department of Pediatrics and Adolescent Medicine, , Aarhus,","place":["Denmark"]},{"name":"Aarhus University Hospital Perinatal Epidemiology Research Unit, , Aarhus,","place":["Denmark"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2018,11,12]]},"reference":[{"issue":"5","key":"2026080508015617700_micit1","doi-asserted-by":"publisher","first-page":"1279","DOI":"10.1093\/humrep\/dei469","article-title":"Factors affecting time to pregnancy","volume":"21","author":"Axmon","year":"2006","journal-title":"Hum Reprod"},{"key":"2026080508015617700_micit2","doi-asserted-by":"publisher","first-page":"574","DOI":"10.1016\/j.envres.2016.03.001","article-title":"Response to letter to the editor regarding \u201cSerum perfluoroalkyl acids and time to pregnancy in nulliparous women","volume":"147","author":"Bach","year":"2016","journal-title":"Environ Res"},{"key":"2026080508015617700_micit3","doi-asserted-by":"publisher","first-page":"535","DOI":"10.1016\/j.envres.2015.08.007","article-title":"Serum perfluoroalkyl acids and time to pregnancy in nulliparous women","volume":"142","author":"Bach","year":"2015","journal-title":"Environ Res"},{"key":"2026080508015617700_micit4","doi-asserted-by":"publisher","first-page":"59","DOI":"10.1186\/s12940-015-0040-9","article-title":"Perfluoroalkyl acids and time to pregnancy revisited: an update from the Danish National Birth Cohort","volume":"14","author":"Bach","year":"2015","journal-title":"Environ Health"},{"issue":"9","key":"2026080508015617700_micit5","doi-asserted-by":"publisher","first-page":"735","DOI":"10.1080\/10408444.2016.1182117","article-title":"Perfluoroalkyl and polyfluoroalkyl substances and measures of human fertility: a systematic review","volume":"46","author":"Bach","year":"2016","journal-title":"Crit Rev Toxicol"},{"key":"2026080508015617700_micit6","doi-asserted-by":"publisher","first-page":"58","DOI":"10.1016\/j.envint.2014.04.010","article-title":"Maternal serum concentrations of per- and polyfluoroalkyl substances and their predictors in years with reduced production and use","volume":"69","author":"Berg","year":"2014","journal-title":"Environ Int"},{"issue":"6","key":"2026080508015617700_micit7","doi-asserted-by":"publisher","first-page":"578","DOI":"10.1093\/oxfordjournals.aje.a008788","article-title":"Smoking reduces fecundity: a European multicenter study on infertility and subfecundity. 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