Three patients are described with defects of blastogenesis and predominantly midline defects. Two were sibs of whom the firstborn were female cephalothoracopagusconjoined twins with multiple predominantly midline defects initially thought to be a sporadic occurrence. A subsequent brother presented with severe hydrocephalus, rhombencephalosynapsis, conotruncal defect, ambiguous genitalia, and unilateral pre-axial polydactyly; an autosomal recessive defect is postulated in this case. Patient 3 had hydrocephalus, small cerebellum, cleft lip and palate, and a large sacrococcygeal teratoma, the better differentiated part of which included ovarian tissue with primordial follicles. The latter may occur in incomplete twinning as a defect of blastogenesis. The mother had had a previous termination of a fetus with hydrocephalus from a different relationship. Counseling was difficult since no examinations were done on the previous fetus and no further investigations of this family could be obtained, but a genetic origin is suspected. Most defects of blastogenesis are sporadic; however, some cases have a genetic cause and ultrasound examinations should be offered in subsequent pregnancies.