Platelet Counts During Pregnancy as Predictor of Subsequent Myeloproliferative Neoplasm

OBJECTIVES: Myeloproliferative neoplasms (MPNs) are associated with increased thrombosis risk, and earlier identification is essential to reduce preventable complications. We investigated whether platelet counts during pregnancy were higher in women diagnosed with MPN after pregnancy than in controls. METHODS: We conducted a nested case-control study of pregnant women in two Danish regions between 2010 and 2017 using registry and laboratory data. The primary exposure was platelet counts during pregnancy, and the primary outcome was a subsequent MPN diagnosis. RESULTS: /L) during pregnancy (325 (IQR 199-419) vs. 219 (IQR 183-254), p = 0.002), and platelet counts > 400 occurred in 29% of cases versus 2% of controls (p < 0.001). Adjusted models showed an estimated 85 higher platelet count in cases, and platelet counts > 400 during pregnancy were associated with an 18-fold increased odds ratio (95% CI: 6-61) for later MPN. MPN status was strongly associated with higher platelet counts before, during, and after pregnancy. CONCLUSION: Elevated platelet counts during pregnancy are strongly associated with subsequent MPN, suggesting that routine prenatal platelet counts may enable early identification of women at risk of MPN.