1. Overview of Twin Pregnancies
Chorionicity and Amnionicity
Chorionicity refers to the number of placental masses (chorions).
Amnionicity refers to the number of amniotic sacs.
Accurate determination of chorionicity and amnionicity in the first trimester (ideally at 11–14 weeks) is paramount because it dictates the surveillance schedule, risk profile, and management plan.
Key Point: The earlier the ultrasound assessment, the more accurately chorionicity can be determined (via identifying the Lambda/Twin-Peak sign in dichorionic twins vs. the T-sign or no visible membrane in monochorionic twins).
Zygosity
Dizygotic (Fraternal): Two separate ova fertilized by two sperm, almost always leading to dichorionic diamniotic (DCDA) twins.
Monozygotic (Identical): One fertilized ovum that splits. Depending on the timing:
Within first 3 days → DCDA
Days 4–8 → Monochorionic diamniotic (MCDA)
Days 8–13 → Monochorionic monoamniotic (MCMA)
After day 13 → Conjoined twins
2. Dichorionic Diamniotic (DCDA) Twins
Definition: Each fetus has its own placenta (which may appear separate or fused) and its own amniotic sac.
Occurrence:
Most common type of twin pregnancy (~70–80% of all twins).
Can be either dizygotic or monozygotic (if the split occurs very early, within 3 days post-fertilization).
Ultrasound Findings:
Lambda (Twin-Peak) Sign: A wedge-shaped extension of placental tissue between the two sacs, best visualized between 10–14 weeks.
Intertwin Membrane Thickness: ≥2 mm (two layers of chorion and two layers of amnion).
Complications:
Generally lower risk compared to monochorionic twins.
Discordant Fetal Growth: Defined as an intertwin estimated fetal weight (EFW) difference of ≥20%.
Twin Anemia–Polycythemia Sequence (TAPS): Possible but rare in DCDA.
Preeclampsia, Gestational Diabetes, Postpartum Hemorrhage (PPH): All twin pregnancies carry higher maternal risks compared to singleton.
Management:
Antenatal Surveillance:
Growth scans every 4 weeks beginning around 20 weeks.
Routine prenatal visits per obstetric guidelines; additional visits if complications arise.
Delivery Timing:
Typically 37–38 weeks if no complications.
Consider earlier if fetal or maternal indications develop (e.g., severe preeclampsia, fetal compromise).
Delivery Mode:
Vaginal delivery is often appropriate if the leading twin (Twin A) is cephalic and there are no obstetric contraindications.
Cesarean delivery is indicated if there are non-cephalic presentations (especially of Twin A), placental issues, or other maternal/fetal indications.
3. Monochorionic Diamniotic (MCDA) Twins
Definition: Twins share a single placenta (monochorionic) but each fetus has its own amniotic sac.
Occurrence:
Always monozygotic (identical twins).
Arises from zygotic division between days 4–8 post-fertilization.
Ultrasound Findings:
T-Sign: A thin membrane inserting directly into the placenta at a right angle, creating a “T” appearance.
Membrane Thickness: <2 mm.
Complications:
Twin-to-Twin Transfusion Syndrome (TTTS): Occurs in ~10–15% of MCDA twins due to imbalanced blood flow via vascular anastomoses.
Twin Anemia–Polycythemia Sequence (TAPS): May occur spontaneously (~3–5%) or after laser treatment for TTTS (~13%). Diagnosed by discordant MCA-PSV (middle cerebral artery peak systolic velocity) measurements without amniotic fluid discrepancy.
Selective Intrauterine Growth Restriction (sIUGR): One twin is growth-restricted due to unequal placental sharing. Subtypes (I, II, III) are defined by the Doppler patterns in the umbilical artery.
Twin Reversed Arterial Perfusion (TRAP) Sequence: Extremely rare; involves an acardiac “parasitic” twin perfused by the pump twin.
Preeclampsia, GDM, PPH: Elevated maternal risks as with all multiples.
Management:
Antenatal Surveillance:
Ultrasound every 2 weeks starting from about 16 weeks to detect TTTS, TAPS, sIUGR, and other issues early.
Fetal echocardiography may be considered to screen for cardiac complications secondary to TTTS.
Interventional Treatments for TTTS or TRAP:
Laser Ablation of Placental Anastomoses for TTTS.
Radiofrequency Ablation (RFA) or Cord Occlusion for severe TRAP sequence or selective IUGR if indicated.
Delivery Timing:
Often 36–37 weeks if asymptomatic and stable.
Consider earlier if signs of TTTS, growth restriction, or other complications worsen.
Delivery Mode:
Vaginal delivery is feasible if Twin A is cephalic and there are no contraindications.
Cesarean delivery for malpresentation, fetal compromise, or obstetric indications.
4. Monochorionic Monoamniotic (MCMA) Twins
Definition: Twins share a single placenta and a single amniotic sac.
Occurrence:
Result of zygotic division between days 8–13.
Extremely rare (~1% of monozygotic twin pregnancies).
Ultrasound Findings:
No Visible Intertwin Membrane: Both fetuses are free-floating in the same amniotic cavity.
Cord Entanglement: Commonly visualized on ultrasound with Doppler studies.
Complications:
Cord Entanglement & Prolapse: Major risk leading to a high rate of fetal demise.
TTTS and TAPS: Possible in MCMA, though the shared amniotic environment is itself the biggest concern.
Fetal Growth Restriction, Congenital Anomalies also possible.
Management:
Antenatal Surveillance:
Very close monitoring due to the risk of sudden fetal compromise from cord accidents.
Non-stress tests (NST) or biophysical profiles (BPP) are often performed several times per week or even daily once hospitalized.
Hospital admission is commonly advised between 24–28 weeks gestation for continuous or near-continuous monitoring in some centers.
Delivery Timing:
Typically around 32–34 weeks to balance prematurity risks against the ever-present hazard of cord entanglement.
Antenatal Corticosteroids (e.g., betamethasone) administered prior to preterm delivery to enhance fetal lung maturity.
Delivery Mode:
Cesarean section is strongly recommended to reduce intrapartum risks.
5. Conjoined Twins
Definition: Incomplete division of the embryonic disc after day 13, resulting in physically connected (conjoined) twins.
Occurrence:
Very rare (~1 in 50,000 to 1 in 200,000 births).
High rate of stillbirth and neonatal mortality.
Ultrasound Findings:
No Distinct Separation: Fetuses remain joined at one or more body regions (thoracopagus, omphalopagus, etc.).
Shared Organs/Structures: Extent determined by detailed ultrasound and MRI.
Complications:
Cardiac and Other Organ Fusion: Prognosis is highly dependent on which organs are shared.
Polyhydramnios or Oligohydramnios may occur depending on the anomaly.
Very high perinatal and neonatal mortality if major vital organs are fused.
Management:
Multidisciplinary Approach:
Maternal–Fetal Medicine, Neonatology, Pediatric Surgery, Cardiology, etc.
Detailed fetal imaging (MRI, targeted ultrasound) to delineate shared anatomy.
Delivery Planning:
Typically by cesarean section due to the anatomical constraints of vaginal delivery.
Timing and mode may be individualized based on maternal condition and fetal viability.
Postnatal Care:
Potential surgical separation if anatomically feasible.
Palliative care in cases of severe anomalies precluding survival.
6. General Complications and Management Considerations in Twin Pregnancies
Regardless of chorionicity:
Preterm Labor and Birth
Twins have a higher risk of preterm birth (PTB). MCMA twins have the highest risk due to cord issues.
Antenatal corticosteroids for fetal lung maturity if early delivery (<34 weeks) is anticipated.
Magnesium sulfate for neuroprotection if delivery <32 weeks is likely.
Fetal Growth Restriction (FGR)
Discordant growth is more common in monochorionic twins due to unequal placental sharing.
DCDA can also have growth discrepancies (often related to different placental implantation sites or maternal factors).
Maternal Risks
Preeclampsia: ~2–3 times more common in twin pregnancies.
Gestational Diabetes: Elevated incidence, likely due to higher placental mass.
Postpartum Hemorrhage (PPH): Greater uterine distension increases the risk.
Anemia: More common due to increased iron demands.
Perinatal Mortality and Morbidity
Elevated in all twin gestations compared to singletons.
Particularly high in MCMA and conjoined twins due to the combination of vascular, cord, and structural risks.
Lifestyle and Supportive Measures
Adequate nutritional intake, including supplementation with iron, folic acid, and sometimes additional calories/protein.
Close monitoring of maternal blood pressure, blood glucose, and hemoglobin levels.
Psychosocial support and counseling, as the incidence of perinatal anxiety and postpartum depression may be higher with multiples.
Antenatal Visit Schedule
DCDA: Similar to singletons initially, with additional ultrasound every 4 weeks from ~20 weeks.
MCDA: More intensive, ultrasounds every 2 weeks starting from ~16 weeks.
MCMA: Very intensive, often requiring inpatient monitoring late in the second or early in the third trimester.
Intrapartum Considerations
Twin A presentation is crucial in deciding the mode of delivery. Cephalic/cephalic is most favorable for a vaginal attempt.
Continuous fetal heart rate monitoring is standard for both twins during labor.
Be prepared for emergent cesarean if Twin B has distress or there is a complication (e.g., cord prolapse).
Postpartum Management
Active Management of the Third Stage (e.g., uterotonics) to reduce PPH.
Monitor closely for postpartum complications (e.g., hemorrhage, infection).
Support for breastfeeding multiples, if desired.
Postnatal follow-up for potential neonatal complications (e.g., anemia, jaundice, growth).
7. Summary Table
Twin Type | Chorionicity & Amnionicity | Surveillance | Delivery Timing | Delivery Mode |
DCDA | 2 chorions, 2 amnions | - Growth scans every 4 weeks from ~20 weeks - Routine OB visits | 37–38 weeks | Vaginal if Twin A cephalic and no obstetric issues |
MCDA | 1 chorion, 2 amnions | - Ultrasound every 2 weeks from ~16 weeks - Monitor for TTTS/TAPS | 36–37 weeks | Vaginal if stable presentation; C-section if indicated |
MCMA | 1 chorion, 1 amnion | - Very frequent monitoring; consider hospital admission by 24–28 weeks | 32–34 weeks | Cesarean section |
Conjoined | Variable (usually monochorionic, single sac or incomplete membrane) | - Detailed imaging (US/MRI) - Multidisciplinary approach | Individualized, often preterm | Cesarean section |
8. Key Take-Home Points
Early Determination of Chorionicity/Amnionicity
Perform a first-trimester ultrasound (11–14 weeks) for the most accurate identification of the Lambda/Twin-Peak Sign (DCDA), T-Sign (MCDA), or no membrane (MCMA).
Monochorionic Twins = Higher Surveillance
Shared placental circulation predisposes to serious complications like TTTS, TAPS, and selective IUGR. Biweekly ultrasounds are critical.
MCMA: A Unique High-Risk Group
Shared placenta + shared amnion = high risk of cord entanglement. Often requires inpatient monitoring and early cesarean.
Conjoined Twins Require Specialized Care
Requires advanced imaging, planning, and a multidisciplinary team. Prognosis depends on the site and degree of fusion.
Maternal Risks Are Increased with All Multiples
Preeclampsia, gestational diabetes, and postpartum hemorrhage are more common. Strict monitoring and preventative measures are essential.
Delivery Decisions
Tailored to each pregnancy’s presentation (fetal positions, growth patterns, maternal conditions).
MCMA twins generally delivered by cesarean around 32–34 weeks.
MCDA and DCDA twins can be delivered vaginally if conditions are favorable and Twin A is cephalic.
Postnatal Management
Vigilance for neonatal complications (e.g., anemia, respiratory difficulties, feeding issues).
Enhanced support for mothers in the postpartum period, given the demands of twins.
Further Reading / References
American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin on Multifetal Gestations.
Royal College of Obstetricians and Gynaecologists (RCOG) Green-top Guidelines on twin and triplet pregnancy management.
International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) Guidelines on the role of ultrasound in twin pregnancy.
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