Mono-mono twins, or “MoMo” twins, are a rare type of identical twins who develop inside a single amniotic sac while sharing a placenta. The term “monoamniotic-monochorionic” indicates the presence of one chorion (the outer membrane surrounding the embryo) and one amniotic sac (the fluid-filled bag). Understanding this unique type of twin pregnancy is crucial, as it comes with heightened risks that every expecting mother ought to be aware of.
How Mono-Mono Twins Form
These twins spring from a single egg fertilized by a sperm that splits into two embryos. A delay in this splitting—typically around a week after conception—can lead to both embryos developing within one amniotic sac. This results in the designation of mono-mono twins.
While the majority of identical twins develop with individual sacs, only about 1% of twin pregnancies are monoamniotic, making the mono-mono configuration exceedingly rare.
It’s important to note that MoMo babies are always identical and share the same sex due to their common genetic makeup.
Diagnosis of Mono-Mono Twins
Routine ultrasound monitoring is a crucial part of twin pregnancies. Doctors check for the presence of a dividing membrane to determine if the twins are in separate sacs. A lack of observable membranes may warrant further examination, as ultrasound remains the only accurate method to identify mono-mono twins.
Early on, mono-mono twins can be misdiagnosed due to a nearly invisible membrane. Subsequent ultrasounds may later reveal a dividing membrane, confirming a different twin type commonly referred to as monochorionic-diamniotic.
Risks of Mono-Mono Twin Pregnancies
Expectant mothers of mono-mono twins should seek care from a doctor specialized in high-risk pregnancies. Consulting with a perinatologist is highly advised, as monoamniotic-monochorionic twins face several health challenges during pregnancy, with a survival rate estimated at 70%.
Umbilical Cord Complications
Each twin connects to the shared placenta through their own umbilical cord, which brings nutrients and oxygen to support their growth. However, as the twins move around within the single amniotic sac, the cords are at risk of intertwining or pressing against each other, potentially cutting off these essential lifelines.
This dangerous scenario increases the likelihood of cord damage or even loss of life for one or both twins if left unaddressed.
Twin-to-Twin Transfusion Syndrome
Mono-mono twins can be vulnerable to twin-to-twin transfusion syndrome (TTTS), where one twin (the donor) effectively provides a blood transfusion to the other twin (the recipient). This often results in the recipient twin receiving more nourishment, while the donor twin may become undernourished and smaller, sometimes leading to anemia.
Diagnosing TTTS in mono-mono twins poses a challenge due to sharing one sac. The assessment relies on monitoring each twin’s physical development to identify any discrepancies.
The Prognosis for Twin-to-Twin Transfusion Syndrome
Risk factors for the recipient twin may include heart failure due to an overload of access blood. Likewise, the donor twin is at risk of organ failure due to insufficient blood flow. A possible intervention is Laser fetoscopy, a brief procedure conducted around 26 weeks, which could aid both twins.
Abnormal Amniotic Fluid Levels
Mono-mono twins may be affected by either low (oligohydramnios) or high (polyhydramnios) levels of amniotic fluid.
A restricted blood supply to one twin can result in inadequate amniotic fluid, leading to restricted movement, reduced bladder capacity, and stunted growth, while excess amniotic fluid can lead to an enlarged bladder and risk of heart failure.
Twin Reversed Arterial Perfusion Sequence
Monochorionic twins may also experience twin reversed arterial perfusion sequence (TRAP sequence). Here, one twin’s heart fails to develop properly while the other twin’s heart compensates for both.
The twin without a heartbeat faces certain death, and the surviving twin may suffer heart failure from the burden of sustaining both. Early delivery or intervention to separate blood flow can enhance the healthier twin’s chances.
Low Birth Weight
Low birth weight often links to lower survival odds, along with heightened risk for health challenges later in life. Mono-mono twins are particularly at risk, nearly quadrupling the likelihood of low birth weight compared to pregnancies with individual placentas.
Weight variations can lead to intrauterine growth restriction (IUGR), further complicating their development.
Preterm Birth
By the 24-week mark, the survival rate for mono-mono twins sits around 75% to 80%. Many may encounter significant complications as early as 26 weeks, necessitating either spontaneous preterm delivery or a scheduled Cesarean section.
Monitoring and Treatment of Mono-Mono Twins
Modern medical technology enables continual monitoring throughout pregnancy. High-resolution ultrasounds, Doppler imaging, and non-stress tests play a vital role in assessing potential issues for mono-mono twins.
Cord complications usually develop gradually, allowing expecting parents and healthcare providers ample time for decision-making, sometimes requiring hospitalization for close observation. Interventions like steroids may be introduced to boost lung development.
Advice
Currently, there are no approved treatments specifically targeting the elevated risks for mono-mono twins. The safest resolution remains delivering the twins, which typically results in premature births.
Physicians must weigh the babies’ in-utero risks against the implications of premature delivery. A planned C-section is typically recommended for MoMo twins, particularly to prevent complications like cord prolapse during delivery.
After 34 weeks, the risk associated with cord entanglement becomes substantial, necessitating delivery. Although one small study indicated vaginal delivery could be safe, the limited number of cases studied, just 29 births, requires caution.
For those learning they are expecting mono-mono twins, the emotional weight can be significant due to the heightened risks involved. Establishing a strong support system will be crucial for navigating this high-stakes pregnancy.
