Please tell me what causes a fetus to develop meningocele?

Multifactorial. Meningoceles fall under a broad category of congenital anomalies called "neural tube defects" "spina bifida" or "split spine" in which the bony spine doesn't fully form over the cord. Can be associated with family history, obesity, diabetes, folate (folic acid) deficiency, high temperature exposure in early pregnancy, use of some anti-seizure medications, a previous history of the same.