We report herein a patient with an MCDA twin pregnancy with ABS noted in one of the twins. To the best of our knowledge, only one previous case of ABS after spontaneous rupture of the dividing membrane in MCDA twins has been reported . However, this previous case report did not describe any antenatal ultrasound findings of ABS, nor did it describe the status of the dividing membrane during the pregnancy. Here, we were able to observe in-utero ABS after early second-trimester rupture of the dividing membrane.
At 12 to 15 weeks, the period during which the placenta is formed, there is a space between the placenta and amniotic membrane. In our patient, the presence of a dividing membrane was confirmed at 13 weeks of gestation, but the dividing membrane could not be seen at 15 weeks of gestation: there was an amniotic membrane rupture between 13 and 15 weeks. As shown in Fig. 4a and b, rupture of the amniotic membrane allows inflow of amniotic fluid and expansion of the space between the amniotic membrane and chorion. Baby B’s upper arm may have entered this newly expanded space. Indeed, we noted a swollen upper arm between the placenta and amniotic membrane on three-dimensional ultrasonography, which showed that the swollen upper arm was actually in the same cavity as baby A. We suggest that baby B’s upper arm pierced the amniotic membrane at another site, causing ABS. We want to emphasize that attention should be paid to the potential for ABS if rupture of the dividing membrane between twins is noted during early gestation, when the chorion and amnion have not yet fused.
Two theories of pathogenesis have been suggested for ABS: exogenous and endogenous theories. The exogenous theory was first described by Torpin  and states that early partial rupture of the amniotic membrane allows parts of the fetal body to enter the space between the amniotic membrane and chorion and they get entrapped by fibrous strands representing amniotic bands. The endogenous theory is endorsed by Streeter, who explains that ABS is caused by developmental abnormalities of the amniotic cavity . Several reports show that ABS is more frequent in monozygotic twin than in dizygotic twin pregnancies [12, 13]. This case was originally MCDA twins and was considered to be prone to develop abnormalities of the amniotic cavity. In the early second trimester, we detected amniotic membrane rupture. Since the amniotic rupture was close to the dividing membrane, the dividing membrane also ruptured creating a single amniotic cavity despite the presence of MCDA twins.
There are case reports describing fetoscopic surgery in patients with ABS, with the intervention conferring a good prognosis [14, 15]. However, fetoscopic surgery can cause complications such as uterine bleeding and preterm rupture of membranes. We conducted frequent ultrasonography, which showed improvement in the swollen upper arm of baby B. We interpreted this as a release of the ABS and therefore did not perform fetoscopic surgery.
A large part of the dividing membrane in this twin gestation was missing at the time of delivery (Fig. 3b). Considering that baby B’s swollen arm improved between 18 and 25 weeks of gestation, the amniotic band that initially entrapped the fetal arm may have been released at some point. We suggest that the dividing membrane in this MCDA twin gestation spontaneously ruptured in the early second trimester. One high-risk maternal-fetal medicine unit reports that the rate of spontaneous rupture is as high as 1.8% . Rupture of the dividing membrane causes a condition equivalent to a monoamniotic twin pregnancy, which is associated with a higher risk of intrauterine fetal demise than diamniotic twin pregnancies, due to the possibility of umbilical cord entanglement . In our patient, there was also the possibility that baby B’s umbilical cord was initially entrapped by amniotic bands. Because we followed our patient closely, with careful attention to umbilical cord blood flow and fetal development, she was able to successfully deliver live newborn twins.
A diagnosis of ABS can be made with two-dimensional ultrasound, but three-dimensional ultrasonography allows for the observation of amniotic bands and any related fetal abnormalities [18, 19]. The addition of three-dimensional ultrasonography is useful for assessing fetal abnormalities . In this case, three-dimensional ultrasonography was able to visualize the swollen fetal arm that was entrapped by an amniotic band. Fetal MRI can also be used to visualize amniotic bands , although this modality is more useful in later pregnancy, when overlapping fetal parts make it difficult to observe details of anatomy on ultrasound. Fetal MRI enables a wide range of images and provides objective evaluation, regardless of the skill of the examiner. In this case, we observed entangled umbilical cords that were free of entrapment by amniotic bands; we were not able to see these features on ultrasonography.
In conclusion, attention should be paid to the potential for ABS if rupture of the dividing membrane between twins is noted during early gestation.