The Swedish Food Agency has conducted recurrent sampling of breastmilk and blood from primiparous women in Uppsala since 1996, in the so-called POPUP study (Persistent Organic Pollutants in Uppsala Primiparas). The Swedish Environmental Protection Agency has funded the study since year 2000. The main aim of the study is to investigate temporal trends of exposure to persistent organic pollutants (POPs) among pregnant and nursing women. Chlorinated paraffins are complex mixtures mainly composed of polychlorinated n-alkanes (PCAs) (Fernandes et al. 2023). They are manufactured by chlorinating paraffinic hydrocarbons of different chain lengths, producing complex mixtures comprising thousands of congeners that vary in carbon chain length and degree of chlorination. Historically, they have been grouped into short-chain (C10–C13, SCCPs), medium-chain (C14–C17, MCCPs), and long-chain (C18C30, LCCPs) chlorinated paraffins. Nowadays, the term chlorinated paraffins is mostly used for the commercial mixtures, while PCAs is used for the more defined polychlorinated nalkanes that are analysed (Fernandes et al. 2023). Chlorinated paraffins are widely used as flame retardants, as plasticizers in flexible polyvinyl chloride (PVC), and as lubricants in metalworking applications. Global production has increased and is estimated to exceed 1 million tonnes annually, with China as the leading producer (Gluge et al. 2016). Owing to their persistence, bioaccumulative properties, and toxicity to aquatic organisms, PCAs are of environmental and public health concern. The short-chain PCAs (SCCPs) were listed as persistent organic pollutants (POPs) under the Stockholm Convention in 2017, and mediumchain PCAs (MCCPs) were listed in 2025 (Stockholm Convention 2025). The aim of this study was to analyse, for the first time, PCAs in serum from first-time mothers in the POPUP cohort and to evaluate temporal trends over the period 1997–2023.