According to traditional theories for the pathogenesis of conjoined twins, diamniotic placentation should not occur. We present an unusual case with diamniotic/monochorionic conjoined twins and discuss possible etiologic hypotheses. The diagnosis of this improbable case was made in the first trimester using ultrasound, which assisted in making an early decision regarding further management.
Twin pregnancy has fascinated obstetric providers for centuries, with conjoined twins receiving attention from the earliest medical recordings. Conjoined twins are hypothesized to develop secondary to the incomplete fission of germinal disc after the 12th day of embryogenesis. This incomplete germinal disc fission would require that conjoined twin pregnancies have monoamniotic and monochorionic membranes. Rare cases of conjoined twins with diamniotic membranes have been reported, which theoretically would be improbable.
Case Report
A 37-year-old woman, gravida 3, para 1, who spontaneously conceived, presented at 9 weeks of gestation for evaluation of her twin pregnancy. Transvaginal ultrasound showed a single intrauterine gestational sac that contained 2 embryos, each of which was surrounded by an amnion ( Figure 1 ), confirming monochorionic diamniotic placentation. The embryos shared a common yolk sac ( Figure 2 ) and were joined periumbilically. The patient’s history included surgical correction of aortic coarctation and subaortic stenosis. Her first pregnancy was uncomplicated and resulted in delivery of a healthy child, who was later diagnosed with autism. During her second pregnancy, dilation and curettage was performed for a blighted ovum at 6 weeks.
A repeated ultrasound ( Figure 3 ) at 11 weeks of gestation confirmed that the fetuses were joined in abdominal region only (omphalopagus). Doppler ultrasound revealed a bifurcated umbilical cord below the conjoined region ( Figure 4 ). She elected termination of pregnancy at this point, and a dilation and curettage was performed without complication. Chromosomal analysis of the products of conception showed no abnormality and normal male karyotype.
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