Objective To evaluate whether the concentration of serum lactate during the diagnosis of postpartum hemorrhage (bleeding ≥500 mL during labor or ≥1000 mL during cesarean delivery) predicts severe hemorrhage (SPPH; blood loss ≥1500 mL at end of labor or in the following 24 h). Methods A prospective cohort pilot study was conducted of women with a vaginal or cesarean delivery from February 2018 to March 2019 who presented with bleeding ≥500 mL measured by the gravimetric method in a reference hospital in San Luis Potosi, Mexico. Venous blood samples were taken for analysis of serum lactate. A receiver operating characteristic curve determined the serum lactate threshold value for SPPH and χ2 test assessed the difference in serum lactate elevation between SPPH and non‐SPPH groups. Lastly, the prognostic capacity between the thresholds was compared. Results SPPH developed in 43.33% of the 30 women in the study group. The best prognostic threshold was 2.68 mmol/L of serum lactate (odds ratio [OR] 17.88, 95% confidence interval [CI] 2.7–16.8, P < 0.001); sensitivity was 0.85 (95% CI 0.55–0.98); specificity was 0.76 (95% CI 0.50–0.93). Conclusion Serum lactate may be a useful prognostic marker for SPPH, more studies are needed to validate these findings.