Background: Colloid cysts, rare benign growths in the third ventricle, can lead to obstructive hydrocephalus, as exemplified by a fatal case in pregnancy due to cyst enlargement. Case Report: Mrs. X, a 30-year-old multiparous woman in her fifth month of pregnancy, was admitted to our hospital unconscious and could not be revived despite CPR efforts. Upon evaluation, her medical history revealed a progressive increase in headache intensity over the preceding three days, followed by vomiting, dizziness, and eventual sudden collapse. Postmortem examination revealed a sizable colloid cyst in the third ventricle, exerting pressure on brain tissue and causing herniation of the cerebellar tonsils. Conclusion: This case emphasizes the importance of thorough investigation into the causes of persistent headaches during pregnancy. Failure to diagnose such insidious yet potentially severe conditions can have life-threatening consequences.