There are 2 similar cases that have been reported:
1) Shipp TD, Genest D, Benacerraf BR. A case of fetal decapitation. J Ultrasound Med 1996 Jul;15(7):535-7.
2) Haider EA, Toi A, Keating S, Kingdom J, Singer S. Fetal survival following decapitation. Ultrasound Obstet Gynecol 2008 Feb;31(2):223-4.