Frequency of Spontaneous Septostomy in Monochorionic Diamniotic Twin Gestation
Spontaneous septostomy in monochorionic diamniotic (MoDi) twin pregnancies is extremely rare, occurring in approximately 1.8% of complicated MoDi gestations referred for fetal therapy, though the true population-based incidence is likely even lower. 1
Incidence Data
The available evidence on spontaneous septostomy frequency is limited:
In a specialized fetal therapy referral center, spontaneous septostomy of the dividing membrane (SSDM) was identified in only 4 out of 217 (1.8%) complicated MoDi multiple gestations without prior invasive procedures. 1
This 1.8% figure represents a highly selected population of complicated MoDi twins already referred for potential fetal therapy (for conditions like TTTS, selective IUGR, or TRAP sequence), not the general MoDi twin population. 1
The true population-based incidence is undoubtedly much lower than 1.8%, as most MoDi twins do not develop complications requiring fetal therapy referral. 1
Multiple case reports and small case series describe spontaneous septostomy as "extremely rare" or "rare" in MoDi twins. 2, 1, 3
Important Clinical Context
Spontaneous septostomy creates a pseudo-monoamniotic gestation, fundamentally changing the risk profile and management approach:
The condition allows both twins to occupy the same amniotic space, leading to potential cord entanglement (reported in approximately half of published cases). 1, 4
Diagnosis is challenging and requires high clinical suspicion, particularly when twins are visualized on the same side of the dividing membrane, when polyhydramnios occurs in the donor's sac despite a collapsed bladder in TTTS cases, or when cord entanglement is identified. 1
Mean gestational age at diagnosis in the largest case series was 19.7 weeks (range 18-20.9 weeks). 1
Critical Management Implications
When spontaneous septostomy is diagnosed or suspected:
Manage as a pseudo-monoamniotic gestation with heightened surveillance, including serial ultrasound monitoring and consideration of inpatient admission for continuous fetal monitoring in the third trimester. 5
Delivery timing is typically earlier than standard MoDi twins due to cord entanglement risk, with reported delivery between 28-34 weeks in recent cases. 5
The condition poses diagnostic and management challenges, particularly when it complicates already high-risk conditions like TTTS, where delayed diagnosis can lead to substantial fetal deterioration. 1