Monochorionic diamniotic conjoined twins: prenatal sonographic diagnosis at 8 weeks.

Abstract

With increasing use of in-vitro fertilization (IVF) and assisted reproductive techniques, the rates of twinning and its complications are rising. Conjoined twins are usually monochorionic monoamniotic (MCMA) and result from postzygotic splitting after Day 131. Monochorionic diamniotic (MCDA) conjoined twins are very rare, with only four cases of their prenatal diagnosis reported in the literature so far. The embryopathology in these cases may be explained by two theories: (1) incomplete fission of the embryonic disc; or (2) secondary fusion of two monovular embryonic discs2, of which the latter seems more convincing in MCDA omphalopagus twins. According to the secondary fusion theory of Spencer2, two embryonic discs fuse in the area of the allantois and yolk sac, which explains the occurrence of two amniotic sacs and a single yolk sac and sharing of the urachus/bladder and colon, abdominal wall defects and exstrophy. A 35-year-old primigravida who conceived by IVF and embryo donation (oocyte donor's age, 25 years; single embryo transfer), presented at 8 weeks for determination of chorionicity. Transvaginal ultrasound findings of a single gestational sac, two amniotic sacs and a single yolk sac suggested a MCDA twin gestation (Figure 1). Two fetal poles, each corresponding to 7 + 4 weeks' gestation, were found fused at the caudal end, giving a ‘V-shaped’ appearance (Figures 1 and 2). On closer examination, the fetuses were found to be joined at the periumbilical region and the umbilical cord bifurcated (forked) below the joined region (Figure 2). These findings were consistent with conjoined MCDA omphalopagus twins. Follow-up examination at 10 weeks confirmed the findings. The parents opted for termination of pregnancy. This is the first case of early diagnosis of MCDA conjoined twins in which the diagnosis was made as early as 8 weeks' gestation. The four other cases reported in literature were diagnosed after 11 weeks' gestation. Early diagnosis is particularly important in today's pre-non-invasive prenatal testing scan era, and this case demonstrates sonographic signs, such as a single large yolk sac, V-shaped appearance of the fetal poles and forked umbilical cord in MCDA twin pregnancy, which should prompt suspicion of conjoined MCDA twins. Confirmation at 10 weeks when fetal movements start may be of value in ruling out a false-positive diagnosis, particularly if termination of pregnancy is planned3. In cases of continuation of pregnancy, antenatal management should be similar to that in any other monochorionic pregnancy, with an additional effort to diagnose structures shared by the conjoined twins, such as bladder, colon and ileum. Cesarean section is the preferred mode of delivery in these cases. An elective separation procedure of the conjoined structures, with or without repair of abdominal wall defect or exstrophy, would be required postnatally, unless there is rupture of omphalocele membrane or stillbirth of one twin, which warrants emergency separation4. Usually, MCDA conjoined omphalopagus twins have a better outcome following surgery than do those that are MCMA. Assisted reproductive techniques, such as intracytoplasmic sperm injection, assisted hatching and micromanipulation, are associated with an increased incidence of monozygosity and conjoined twins due to reduced integrity of the zona pellucida, herniation of the blastomeres and splitting of the embryo5. This and the fusion theory explain the occurrence of MCDA conjoined twins in the case presented here. In conclusion, conjoined twins can occur in the setting of MCDA pregnancy and early sonographic diagnosis in these cases is possible which would aid in counseling the parents and managing the pregnancy.