A Report of Dizygous Monochorionic Twins

fertilization. The twins also had blood chimerism (the presence of cells derived from more than one genetically distinct zygote); in both twins, there was a predominance of male-derived lymphocytes, and as a result, the initial findings of zygosity studies per- formed on DNA extracted from the blood were consistent with monozygosity. Subse- quent detailed investigation showed the twins to be dizygous. The evidence from this case is inconsistent with the doctrine that all monochorionic twins are monozygous. A 48-year-old woman conceived twins by in vitro fertilization without intracytoplasmic sperm injection and with the use of donor oocytes. Three oocytes were successfully fer- tilized, cultured to the blastocyst stage of development, and placed in the recipient's uterus. An ultrasound scan obtained at six weeks of gestation was reported to show a monochorionic, diamniotic twin pregnancy and a third sac with a nonviable fetus. A sub- sequent scan obtained at 12 weeks of gestation showed a viable twin pregnancy, with findings indicative of monochorionic diamniotic twinning, including a thin intertwin membrane, a T -shaped insertion of the membrane, and absence of the lambda sign (a tri- angular projection of placental tissue extending into the base of the intertwin membrane, which is visible at this stage of gestation in dichorionic twins). 4 Ultrasound examination at 20 weeks of gestation was consistent with the earlier evidence of monochorionicity, but the twins appeared to be discordant for sex. The pregnancy was otherwise uncom- plicated, and at 37 weeks of gestation, a healthy boy (weighing 2114 g) and a healthy girl (weighing 2183 g) were delivered. No evidence of sexual ambiguity was present. Both twins had type O Rh-positive blood and negative Coombs' tests. Pathological ex- amination showed a monochorionic, diamniotic placenta. Because of this finding, blood samples were obtained from each twin at one week of age for zygosity studies and at three months for cytogenetic studies. Evaluation of the twins at five months of age showed unremarkable results on physical examination and normal, sex-appropriate ex- ternal genitalia. Abdominal ultrasound examination of the female twin confirmed the presence of a uterus and ovaries. Peripheral-blood samples and skin-biopsy specimens were obtained at five months of age for additional cytogenetic and zygosity studies. Oral informed consent for clinical evaluation of the children and written consent for publica- tion of the results were obtained from the parents. Tissue samples were not available from either of the biologic parents. i