This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Primary acalvaria is an extremely rare malformation characterized by the absence of the flat skull bones of the brain, dura mater, and scalp muscles. The skull base and facial features are fully formed and usually appear normal. The cause of acalvaria is still unknown. Acalvaria can be distinguished from anencephaly, the most common differential diagnosis, by the presence of a layer of skin overlying the brain matter and normal cerebral hemispheres.  This malformation is most often lethal at birth due to other associated anomalies or to trauma during delivery, but a few surviving infants have been reported. Prenatal diagnosis via transvaginal ultrasound and/or magnetic resonance imaging is critical for better pregnancy management. The initial treatment is conservative, mainly aimed at supportive care and management of any associated anomalies, if present.  Infants with acalvaria are managed conservatively in the newborn period because spontaneous bone growth has been seen in some newborns with other skull abnormalities, such as aplasia cutis congenita. Skull reconstruction by bone grafting and cranioplasty at school age has been discussed in the literature. 
For more information, visit GARD.