Identical twins, also known as monozygotic twins, are formed when a single fertilized egg splits into two embryos. This results in twins who share 100% of their DNA and are always the same sex. While most identical twins share a placenta, some rare forms exist where each twin has their own placenta and amniotic sac. The rarest type of identical twins are called monoamniotic-monochorionic or “mono mono” twins.
What are monoamniotic-monochorionic (mono mono) twins?
Monoamniotic-monochorionic twins, or “mono mono” twins, share the same amniotic sac but have separate umbilical cords. This occurs in around 1 in 10,000 identical twin pregnancies, making it very rare. In mono mono twins, a fertilized egg splits around 8-12 days after conception. Normally, identical twins split around days 13-15. Splitting earlier than usual results in twins sharing an amniotic sac but developing separate placentas and umbilical cords.
Sharing an amniotic sac comes with risks. With no dividing membrane, the fetuses’ umbilical cords can become entangled. Mono mono twins are also at risk for twin-to-twin transfusion syndrome (TTTS). Despite risks, most mono mono pregnancies result in healthy babies with careful monitoring.
What causes mono mono twins?
Doctors don’t know exactly why mono mono twins occur. Early embryo splitting likely plays a role. Around day 4 after fertilization, embryos transition from one cell into a cluster of cells called a morula. At day 5-6, the morula becomes a blastocyst – a fluid-filled structure containing an inner cell mass that will form the fetus. The inner cell mass then splits into two, forming identical twins.
In mono mono twins, this split occurs extremely early – around day 8-12 vs day 13-15 in most identical twins. Researchers believe this explains the shared amniotic sac. Later splitting allows time for separate sacs to form. With very early splitting, two embryos share one sac. The cause of early splitting is unknown but may relate to embryo development rates or environmental factors. Genetics likely play a small role.
Signs and symptoms of mono mono twins
Monoamniotic twins are usually diagnosed by ultrasound around 11-14 weeks gestation. Signs may include:
- A single placenta
- No dividing membrane in the amniotic sac
- Two umbilical cords
- Two yolk sacs early in pregnancy
Other signs like measuring large for gestational age may occur later. Symptoms specific to mono mono twins include:
- Cord entanglement and knots
- Difficulty determining chorionicity (amnionicity is clear)
- Higher risk for twin-twin transfusion syndrome (TTTS)
TTTS causes abnormal blood flow between twins. Symptoms include chronic twin imbalances, such as one twin appearing much smaller. TTTS requires immediate treatment to prevent heart failure, vascular disruption, or death.
Risks of mono mono twins
Monoamniotic twins face higher risks than twins with separate sacs. Risks include:
- Cord entanglement – Cords can wrap tightly around each other, cutting off oxygen and blood flow.
- Cord compression – The amniotic sac provides little space, allowing cords to compress.
- Twin-twin transfusion syndrome (TTTS) – Shared circulation can disrupt blood flow.
- Preterm birth – 60% are born before 32 weeks gestation.
- Birth injury – Entanglement can cause injury during delivery.
- Perinatal mortality – Overall rates are historically around 50%, mainly due to cord accidents.
Modern medicine has reduced risks significantly. Perinatal mortality rates are now around 10-15% in experienced care centers. Still, mono mono twins are considered high risk and require close monitoring.
Complications of mono mono twins
Mono mono twins can experience similar complications as other monozygotic twins, plus added risks related to their shared sac:
- Cord entanglement, compression, or prolapse
- Twin-twin transfusion syndrome (TTTS)
- Preterm premature rupture of membranes (PPROM)
- Preterm labor and birth
- Birth injury due to entanglement
- Fetal demise of one or both twins
- Growth discordance – one twin is smaller
- Neurological impairment from hypoxia before birth
Complications like prematurity can affect long-term outcomes. Hypoxia before birth also raises the risk of brain damage and cerebral palsy. However, most mono mono twins are born healthy with proper medical care.
Treating complications in mono mono twins
Doctors use the following treatments for mono mono twin complications:
- Cord entanglement – No treatment; emergency C-section if found during delivery.
- TTTS – Laser ablation surgery to seal off connecting blood vessels.
- PPROM – Steroids for lung development; antibiotics; bed rest.
- Preterm birth – Steroids; magnesium sulfate; limiting physical activity.
- Birth injury – Allow vaginal delivery if cords separate; emergency C-section if entangled.
- Growth discordance – Fetal echocardiogram; ultrasound; selective reduction if severe.
Doctors also carefully monitor mono mono twins for signs of complications. This allows early intervention when needed.
Diagnosing mono mono twins
Monoamniotic twins are typically diagnosed on an ultrasound around 11-14 weeks gestation. Signs include:
- Single placenta
- No dividing membrane in amniotic sac
- Two umbilical cords entering one sac
- Two yolk sacs visible early on
Doctors may suspect monoamniotic twins earlier based on hCG levels. However, ultrasound provides definitive diagnosis. It accurately shows a single sac and two cords, which occurs in 1% of identical twin pregnancies.
After diagnosis, frequent ultrasounds help monitor for cord entanglement, TTTS, and growth problems. Fetal echocardiograms may also detect heart issues related to TTTS. Testing for birth defects can determine if selective reduction is needed.
Mono mono twins ultrasound
Ultrasound is the main diagnostic tool and provides the classic signs:
- Single gestational sac – Unlike diamniotic twins with two sacs.
- Absent dividing membrane – The amniotic sac contains two fetuses without a dividing septum.
- Two yolk sacs – Visible early on before the membrane disappears.
- Two umbilical cords – Inserting into a shared placenta and amniotic sac.
3D color Doppler imaging shows cord entanglements and inserts. It also helps track twin growth, amniotic fluid, placenta share, and blood flow for TTTS signs.
Table summarizing mono mono twin ultrasound findings
| Ultrasound Feature | Finding in Mono Mono Twins |
|---|---|
| Number of gestational sacs | One shared sac |
| Dividing membrane | Absent |
| Yolk sacs | Two present early in pregnancy |
| Umbilical cords | Two cords seen inserting into shared placenta |
Regular ultrasounds continue throughout pregnancy to monitor issues and growth.
Mono mono twin belly
In a mono mono twin pregnancy, the mother’s belly often measures larger than expected compared to diamniotic twins. This occurs because:
- No dividing membrane allows twins to grow side-by-side.
- Twins share a cramped single sac instead of having their own space.
- Later pregnancy may show clear outlines of two babies.
- Abnormal growth due to TTTS can enlarge one twin.
Mothers pregnant with monoamniotic twins often have a larger than average belly. However, belly size is not a reliable sign. Ultrasound is needed for diagnosis.
Mono mono twins perinatal mortality rate
Historically, monoamniotic twins suffered a perinatal mortality rate around 40-60%. This has improved significantly with modern fetal monitoring and neonatal care:
- 1960s – Perinatal mortality was 60%
- 1980s – Rates fell to 26%
- 1990s – Further declined to 11%
- 2000s – Current rates are around 10-15%
Higher mortality is mainly due to cord accidents. Entanglement and compression can cut off oxygen and blood flow. TTTS and preterm birth also contribute. With intensive monitoring, most mono mono twins now survive and thrive long-term.
Table of mono mono twin perinatal mortality rate by decade
| Decade | Perinatal Mortality Rate |
|---|---|
| 1960s | 60% |
| 1980s | 26% |
| 1990s | 11% |
| 2000s | 10-15% |
Mono mono twins at 20 week ultrasound
The 20 week anatomy scan gives important information about monoamniotic twins:
- Shared sac – Confirms mono amnionicity vs diamniotic.
- Two cords – Inserting into one placenta.
- Entanglement – potential cord issues.
- Anatomy – Screens twins for birth defects.
- Growth – Checks for twin imbalances.
- Doppler – Assesses blood flow for TTTS.
This scan provides a detailed look at mono mono twins. It screens for developing complications like entanglement and TTTS. Doctors also use it to determine if selective reduction is required for major defects.
Mono mono twins selective reduction
Selective reduction aborts one twin in a high risk pregnancy. It is sometimes done in monoamniotic twins when:
- One twin has a severe defect
- Severe twin-twin transfusion syndrome (TTTS)
- Severe growth problems in one twin
Selective reduction lowers risks for the remaining healthier twin. However, the procedure also comes with risks like premature birth. The decision involves balancing risks of continuing the pregnancy vs selective reduction. Parents must weigh complex emotional and medical factors.
Delivering mono mono twins
Monoamniotic twins are high risk for labor complications. Doctors often deliver them via scheduled C-section around 32-34 weeks or sooner if complications arise. Reasons for planned early delivery include:
- Preventing stillbirth from cord accidents
- Avoiding emergency C-section for entangled cords
- Preventing death or brain damage if cords compress during labor
- Reducing the risk of preterm birth complications
Vaginal delivery may be considered if cords are clearly separated and TTTS risk is low. During delivery, doctors carefully monitor cords for entanglement. If detected, an emergency C-section is immediately performed.
Mono mono twins outcomes
Most monoamniotic twins have good outcomes when given proper medical care:
- Around 60% are born preterm.
- With intensive monitoring, perinatal mortality is around 10-15%.
- About 20% have neurological impairment related to prematurity.
- Major birth defects affect 10%.
- With modern interventions, over 85% survive infancy.
- Long-term issues like cerebral palsy occur in less than 10%.
Outcomes greatly depend on access to specialized high-risk prenatal care. Early detection and treatment of complications improve survival and reduce disability. Overall, most mono mono twins have positive health outcomes today.
Caring for mono mono twins
Caring for monoamniotic twins requires:
- Referral to a maternal-fetal medicine (MFM) specialist
- Frequent ultrasounds to monitor cords, growth, amniotic fluid, and blood flow
- Any needed fetal interventions such as laser ablation for TTTS
- Steroid shots and avoiding early delivery as long as possible
- Planned early delivery around 32-34 weeks via C-section
- Pediatric team ready at delivery to resuscitate if needed
- Admitting babies to the NICU until stable, feeding well, and gaining weight
- Providing any needed newborn care such as respiratory support
With a skilled healthcare team, most mono mono twins survive and grow up healthy, despite higher risks.
Mono mono twins success stories
Most monoamniotic twin births result in healthy babies with positive long-term outcomes. Here are some real life success stories:
Case 1: Susan was shocked to learn her twins were mono mono at 14 weeks. She transferred to a maternal-fetal medicine practice. With close monitoring, Susan safely delivered her twins at 33 weeks. The babies spent 2 weeks in the NICU mastering feeding skills. Now 2 years old, the twins are hitting all their milestones.
Case 2: Kim found out she had mono mono twins at 10 weeks. At 18 weeks, laser ablation treated mild TTTS. Her boys were born early at 31 weeks but had no other complications. After a short NICU stay, the twins went home and are now thriving as energetic toddlers.
Case 3: Vanessa was originally told she had twins sharing a placenta. At 16 weeks, imaging revealed her twins were actually mono mono. Born at 34 weeks via planned C-section, both newborns transitioned to breastfeeding quickly. Now, Vanessa is enjoying life with her healthy 4-month-old girls.
Conclusion
Monoamniotic or “mono mono” twins are the rarest type of identical twins, occurring in 1% of such pregnancies. Mono mono twins face higher risks related to cord accidents and twin-twin transfusion syndrome. However, with skilled care, over 80% are born healthy and survive infancy. Modern medicine has greatly reduced risks and improved outcomes. While still considered high risk, most mono mono twins go on to live full, normal lives.