# Forever chemical and early development: what a 2026 stem-cell study means if you train > Karolinska reports a forever chemical altered early germ-cell models at levels seen in some pregnant women's blood. What active adults who may conceive should know. Published 2026-11-13 · Updated 2026-11-13 · revns editorial A **November 2026** study from **Karolinska Institutet**, published in **Human Reproduction Open**, reports that **PFOS**, a regulated “forever chemical” in the [PFAS](/dictionary/pfas) family, **slowed growth** and **changed gene activity** in **stem-cell models** of very early human embryo and **primordial germ-cell** development at **7 nanomoles per litre** — a level Karolinska says **matches blood serum PFOS in some pregnant women** in Sweden’s **SELMA** cohort. **You cannot use this paper to diagnose yourself**, and it does **not** tell you to stop training. The practical takeaway for active adults who may conceive is to **treat everyday PFAS exposure as worth trimming where it is easy**, **keep evidence-based exercise and sleep habits**, and **talk to a clinician** about fertility or pregnancy planning rather than chasing unproven detox products. **TL;DR** | Finding | Detail from Karolinska / the paper | What you do now | | --- | --- | --- | | Design | Human pluripotent stem cells in a 3D model of post-implantation development | Lab model only; not a clinical trial in pregnant women | | Chemicals tested | PFOS, PFOA, PFHxS | PFOS had the strongest effect in this setup | | Lowest PFOS dose | **7 nmol/L**, matched to pregnant women’s serum in SELMA | Shows effects at environmentally relevant levels in the model | | Growth | Embryo-like structures grew **more slowly** and stayed **smaller** over **four days** vs controls | Association in dishes; long-term human outcome unknown | | Genes | **746 genes** differed from controls at that dose; development and signalling pathways repressed | Germ cells still formed, but expression patterns shifted | | Training | Not studied | No reason in this paper to cut workouts before conception | | Limits | Model lacks full womb environment | Authors call for animal and human follow-up | ## What did the Karolinska study find? Karolinska Institutet announced the work on **13 November 2026**. The peer-reviewed article **“PFOS disrupts the development of early germline and surrounding embryonic soma at environmentally relevant exposure levels”** appeared in **Human Reproduction Open** (doi:[10.1093/hropen/hoag099](https://doi.org/10.1093/hropen/hoag099)), with **João Pedro Alves-Lopes** as lead author alongside collaborators including **Volker M Lauschke** and **Pauliina Damdimopoulou**. The team used **human pluripotent stem cells** to build a **three-dimensional model** of stages that occur **soon after embryo implantation**, including **primordial germ cells** — the cells that later become eggs or sperm. They exposed the model to three legacy PFAS compounds already under regulation but still persistent in the environment: **PFOS**, **PFOA**, and **PFHxS**. **PFOS** stood out. During **four-day** experiments: - **Embryo-like structures** grew **more slowly** and remained **smaller** than untreated controls. - That happened even at **7 nmol/L**, the **lowest concentration tested**. - Karolinska states that concentration was chosen because it corresponds to **PFOS levels in blood serum from pregnant women** in the **Swedish Environmental Longitudinal Mother and Child, Asthma and Allergy (SELMA)** study. - **Primordial germ cells still formed** after PFOS exposure, but **gene expression** differed from controls. At the baseline dose, activity of **746 genes** differed, with **repression of biological processes** involved in **development and cell signalling**. Karolinska quotes Alves-Lopes: even **low** PFOS concentrations **may perturb fundamental processes** of early embryonic and germ-cell development in this platform, underlining that **everyday environmental exposures** could matter for **reproductive health**. That is a hypothesis the lab supports; it is **not** the same as proving harm in pregnancies. The institute is explicit about limits. The model captures key aspects of **early post-implantation development** but **does not fully recreate a human embryo or the complex uterine environment**. **Complementary studies**, including **animal models** and **human epidemiology**, are needed to learn whether these dish findings predict real-world outcomes, including for the many PFAS compounds still poorly characterised. Medical Xpress syndicated the story on the same day. Its summary matches Karolinska’s numbers. We could not fetch the Medical Xpress page in this environment because of a **403** response; all factual claims here trace to **Karolinska’s press release** and the **journal citation**. ## What are PFAS, and why do trainers hear about them? [PFAS](/dictionary/pfas) (**per- and polyfluoroalkyl substances**) are a large class of **synthetic chemicals** that resist heat, water and oil. Manufacturers have used them in **waterproof jackets**, **food packaging**, **cosmetics**, **firefighting foam**, and **non-stick cookware**. They break down **very slowly**, so they accumulate in **water, soil, fish and human blood**. Regulators have restricted **PFOS** and **PFOA** in many countries, but **legacy contamination** persists. The **US EPA** states that because PFAS are **widespread and persistent**, many people have **low levels** in blood, and that **scientific studies link some PFAS exposures to harmful health effects**, while **thousands** of related chemicals remain **hard to study**. EPA does **not** claim any single blood level predicts infertility. For people who train, exposure routes often overlap with **nutrition and recovery habits**: - **Drinking water**, especially where PFAS has entered wells or supplies. - **Prepared and packaged foods**, including some grease-resistant wrappers. - **Cookware** when **non-stick coatings** are scratched or overheated. - **Stain-proof textiles** on gym bags or outdoor kit (usually a smaller slice of total exposure). None of that means you must panic over a single takeaway cup. It means **background exposure is common**, and **new lab studies** like this one push researchers to ask what **early development** might see at **real-world concentrations**. ## Does this change exercise if you are trying to conceive? **Not directly.** The Karolinska paper does not measure **workouts**, **macros**, or **sleep**. Current **preconception and pregnancy activity guidance** still favours **regular moderate activity** for most people, with clinician input for high-risk histories. If you already follow revns-style logging, keep doing what supports fertility **lifestyle factors with stronger human evidence**: adequate **protein** ([guide](/blog/how-much-protein-per-day)), **sleep** ([guide](/blog/how-much-sleep-do-you-need)), **stress management**, and **consistent training** you can recover from. See [exercise in pregnancy and postpartum](/blog/exercise-in-pregnancy-and-postpartum-general-guidance-red-flags) for red flags once you are pregnant. This PFOS story adds a **different axis**: **environmental chemicals** that you **partially** control. It does **not** justify extreme diets, sauna protocols marketed for “detox”, or stopping sport without medical reason. ## What can you do this week to lower routine PFAS exposure? Think **harm reduction**, not perfection. EPA’s consumer materials focus on **meaningful steps** rather than zero exposure, which is unrealistic. **Hydration after sessions** - If your tap water is on a **public system**, read your utility’s **annual water quality report** for PFAS testing. In the United States, newer federal rules push suppliers toward **lower PFAS limits**; elsewhere, check your national or regional water authority. - If you use a **private well**, ask your health department about **PFAS testing**. - A **certified filter** labelled for PFAS reduction can help when water is a known source. See [how much water to drink daily](/blog/how-much-water-should-you-drink-daily) for volume targets; the **container and source** matter here. **Food at home and on the go** - Favour **whole foods** you prepare when you can. Grease-resistant **fast-food packaging** can be a PFAS source; you do not need to ban eating out, but **home-cooked dal, beans, tofu, eggs, fish, vegetables and grains** (vegetarian or not) cut wrapper contact. - Rotate **high-protein vegetarian plates** ([guide](/blog/high-protein-vegetarian-meals-indian-and-global)) without assuming plastic packaging is harmless; rinse and store in **glass or steel** when practical. **Kitchen kit** - Replace **scratched non-stick pans**. Avoid **empty pre-heating** non-stick above medium heat; use **cast iron, steel or ceramic** for high-heat searing. - Do not assume “PFOA-free” labels mean **PFAS-free**; many replacements are still fluorinated. Simple cookware lasts through years of meal prep. **Tracking** - Note **water source**, **takeaway frequency**, and **cycle or conception plans** in your log. An [AI fitness coach](/features/ai-fitness-coach) can read patterns you already record; it **does not** interpret blood PFAS results. These steps align with EPA’s aim to reduce **predictable** exposure. They are **not** a guarantee of any fertility outcome. ## How does this fit next to menstrual cycle and fertility tracking? Many athletes already tune training to the [menstrual cycle](/blog/training-around-your-menstrual-cycle-what-evidence-supports). **Cycle length, symptoms and load** stay useful; this study does **not** replace them. If you are **charting ovulation** or going through **fertility treatment**, bring **environmental health** questions to your **reproductive specialist**, not social media. A stem-cell paper cannot tell you whether **your** PFOS level caused **your** timeline. Population studies of PFAS and fertility exist, but they show **mixed associations**; this new model is meant to **speed up screening** of chemicals, not to hand you a home test. Men who train should notice the **germ-cell** angle: the Karolinska model includes **precursors of sperm and eggs**, and PFOS shifted gene expression in those cells at the tested dose. That still does **not** mean every man with detectable PFOS is infertile, and it is **not** proof about any one person's fertility. ## When should you talk to a clinician? Book a **preconception visit** if pregnancy is on the horizon within the next year, especially if you are **over 35**, have **irregular cycles**, **known endocrine conditions**, or **prior pregnancy loss**. Ask what **blood work**, **supplements**, or **vaccinations** make sense for you. This article cannot replace that visit. Seek **urgent or emergency care** for pregnancy red flags listed in [exercise in pregnancy guidance](/blog/exercise-in-pregnancy-and-postpartum-general-guidance-red-flags): bleeding, severe pain, reduced fetal movement, and similar. If **trying to conceive** is affecting your **mood** — anxiety, low mood, or thoughts of self-harm — contact your **GP, midwife or reproductive counsellor**, or your **local crisis line** or emergency number. No app, including revns, provides crisis care or mental health treatment. ## Related - [Exercise in pregnancy and postpartum: general guidance and red flags](/blog/exercise-in-pregnancy-and-postpartum-general-guidance-red-flags) - [Training around your menstrual cycle: what the evidence supports](/blog/training-around-your-menstrual-cycle-what-evidence-supports) - [How much water should you drink daily?](/blog/how-much-water-should-you-drink-daily) - [High-protein vegetarian meals: Indian and global ideas](/blog/high-protein-vegetarian-meals-indian-and-global) - [How to track mood and energy next to training](/blog/how-to-track-mood-and-energy-next-to-training) This is general training guidance, not medical advice. It does not diagnose infertility, pregnancy complications or PFAS-related illness, and it does not replace care from your GP, fertility specialist or maternity team. ## Sources - [Karolinska Institutet. Forever chemical disturbs early development in stem cell model. Press release, 13 November 2026](https://news.ki.se/forever-chemical-disturbs-early-development-in-stem-cell-model) - [Pérez Casasús S et al. PFOS disrupts the development of early germline and surrounding embryonic soma at environmentally relevant exposure levels. Human Reproduction Open 2026; doi:10.1093/hropen/hoag099](https://doi.org/10.1093/hropen/hoag099) - [US EPA. PFAS Explained](https://www.epa.gov/pfas/pfas-explained) - [US EPA. Our current understanding of the human health and environmental risks of PFAS](https://www.epa.gov/pfas/our-current-understanding-human-health-and-environmental-risks-pfas) ## Questions ### Did the study prove PFOS harms human pregnancies? No. The Karolinska team used human pluripotent stem cells to build three-dimensional models of early post-implantation development in the lab. PFOS changed growth and gene activity in those models. The authors state the platform does not fully recreate a human embryo or the uterine environment, and more animal and epidemiological work is needed. ### At what PFOS level did effects show up? The lowest concentration tested was 7 nanomoles per litre. Karolinska Institutet chose that level because it corresponds to PFOS measured in blood serum from pregnant women in the Swedish SELMA pregnancy cohort. Effects on embryo-like structures appeared even at that concentration over four days of exposure. ### Should I stop exercising if I am trying to conceive? No change from current guidance. NHS and CDC materials still support appropriate physical activity before and during pregnancy for most people. This study is about environmental chemical exposure, not training load. Keep your usual exercise habits unless your clinician advises otherwise. ### What are PFAS and where do people meet PFOS? PFAS are long-lasting synthetic chemicals used in water-repellent fabrics, some food packaging, non-stick cookware and many industrial products. PFOS is one regulated legacy compound. The US EPA notes PFAS are widespread in water, food and consumer goods, which makes full avoidance impossible. ### Can I get my PFOS level tested through revns? No. revns logs training, sleep, food and recovery. It does not run blood tests or interpret lab results. If you want PFAS blood testing, ask your clinician whether it is available and what it would mean for you. ### Does this study affect men who train? The model includes primordial germ cells, the precursors of both eggs and sperm. PFOS altered gene expression in those cells at the tested concentration. The paper does not report outcomes in living men or women, so you should treat it as a warning sign for research, not proof that your training diet caused a fertility problem. --- Canonical: https://revns.com/blog/pfos-pfas-early-development-stem-cell-study-2026 Index of everything: https://revns.com/llms.txt revns gives training guidance, not medical advice.