Background <p>Amniotic fluid embolism (AFE) is a rare but highly fatal obstetric emergency. Its pathophysiology remains incompletely understood, and reliable early warning signs are lacking. AFE most commonly occurs during labor or the immediate postpartum period, whereas spontaneous AFE occurring in the second trimester with intact membranes is exceedingly rare. Potential precipitating factors in such cases remain poorly defined.</p> Case presentation <p>We report a case of a 30-year-old primigravida in whom a large subchorionic hematoma (SCH) was identified on ultrasonography at 13⁺¹ weeks of gestation. The patient remained asymptomatic and was managed conservatively with oral dydrogesterone. At 16⁺⁶ weeks of gestation, she abruptly developed altered consciousness, seizures, and vaginal bleeding, and was admitted with acute hypotension, hypoxemia, and severe coagulopathy. Following comprehensive assessment by the multidisciplinary team , the diagnosis was considered to be spontaneous AFE. Emergency hysterotomy was performed in conjunction with massive transfusion and supportive care, resulting in complete clinical recovery.</p> Conclusion <p>Based on this case, we propose the hypothesis that a large, persistent SCH may represent a state of anatomical vulnerability at the maternal–fetal interface and may be associated with spontaneous amniotic fluid embolism in the second trimester. In patients with large SCH, acute cardiopulmonary or neurologic symptoms accompanied by severe coagulopathy should prompt early consideration of spontaneous AFE and immediate multidisciplinary management. Further cases and mechanistic studies are needed to clarify the potential relationship between SCH and spontaneous AFE.</p>