Fetal arachnoid cysts can generally be detected by ultrasound after 20 weeks of pregnancy, but not absolutely, the exact time varies from person to person. Fetal arachnoid cyst is arachnoid membrane local wrapping cerebrospinal fluid accumulation therein and formed, and intracranial ventricular system without communication, is a benign lesion of abnormal development in the embryonic period, generally can be detected by ultrasonography after 20 weeks of pregnancy, the cysts are relatively small, there is no combination of other developmental malformations or lesions, with the growth of fetal age, the cysts will be slowly absorbed on their own, and the pregnancy can be continued. Through the ultrasound examination of the cyst is relatively large, there is obvious pressure on the brain tissue, the cranium produces neurological symptoms, so that the cranium changes, with the increase of gestational weeks, there is no resorption or incomplete resorption, the cyst is obviously enlarged, it is necessary to carefully consider whether to continue the pregnancy. Fetal arachnoid cysts are detected during pregnancy, it is necessary to do a regular review of the pregnancy, and timely decision on whether to continue the pregnancy.