2.3 Multiple Gestation: Placentation, Chorionicity & Complications

Embryology: Zygosity vs. Chorionicity & Amnionicity

Understanding the anatomical and vascular architecture of multifetal gestations is essential for inpatient obstetric nursing. Chorionicity (the number of chorionic membranes/placentas) is the primary determinant of pregnancy risk, morbidity, and management, rather than zygosity (genetic identity).

                  [ MONOZYGOTIC CLEAVAGE TIMELINE & PLACENTATION ]

  Cleavage Days 0 – 3          Cleavage Days 4 – 8          Cleavage Days 8 – 12         Cleavage >13 Days
     (Morula Stage)             (Blastocyst Stage)           (Embryonic Disc)           (Incomplete Disc)
           │                            │                            │                          │
           ▼                            ▼                            ▼                          ▼
      [ Di / Di ]                  [ Mo / Di ]                  [ Mo / Mo ]                [ Conjoined ]
  • Dichorionic / Diamniotic   • Monochorionic / Diamniotic • Monochorionic / Monoamniotic • Shared organs
  • 2 Placentas, 2 Amnions     • 1 Placenta, 2 Amnions      • 1 Placenta, 1 Amnion      • Thoracopagus,
  • Thick membrane (>2 mm)     • Thin membrane (<2 mm)      • No dividing membrane        Omphalopagus
  • Lambda (λ) sign            • T-sign                     • Cord entanglement (>95%)  • Very rare (<1:50k)
  • ~25–30% of MZ twins        • ~70–75% of MZ twins        • ~1–2% of MZ twins

Zygosity & Chorionicity Types

1. Dizygotic Twins (Fraternal, ~70% of all twins): Fertilization of two distinct ova by two separate sperm. Genetically non-identical. Always Dichorionic / Diamniotic (Di/Di). Each fetus possesses its own distinct placenta and amniotic sac. (While adjacent placentas may physically fuse, their vascular systems remain completely separate with no communicating anastomoses).
2. Monozygotic Twins (Identical, ~30% of all twins): A single ovum fertilized by a single sperm that subsequently divides. The timing of cleavage dictates placentation:
   • Days 0–3 (Morula stage): Cleavage occurs prior to inner cell mass differentiation. Results in Dichorionic / Diamniotic (Di/Di) twins (~25% to 30% of monozygotic gestations). Two separate placentas, two separate chorions, two amniotic sacs.
   • Days 4–8 (Blastocyst stage): Inner cell mass divides while the trophoblast has already formed. Results in Monochorionic / Diamniotic (Mo/Di) twins (~70% to 75% of monozygotic gestations). Single shared placenta, one chorion, two separate amniotic sacs. Shared placental vascular connections create high risk for hemodynamic complications.
   • Days 8–12 (Implanted embryonic disc): Division occurs after the amnion has formed. Results in Monochorionic / Monoamniotic (Mo/Mo) twins (~1% to 2% of monozygotic gestations). Single shared placenta, single shared amniotic sac with no dividing membrane. Severe risk of umbilical cord entanglement and knotting.
   • Day >13: Incomplete embryonic disc division results in Conjoined Twins.

First-Trimester Sonographic Membrane Signs

• Lambda (λ) / "Twin Peak" Sign: A triangular wedge of chorionic tissue extending into the base of the inter-twin dividing membrane (>2 mm thickness). Confirms Dichorionic (Di/Di) placentation.
• T-Sign: The thin dividing amniotic membrane (<2 mm thickness) attaches directly to the single placental surface at a sharp 90-degree angle without intervening chorionic tissue. Confirms Monochorionic (Mo/Di) placentation.

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Monochorionic Vascular Complications

Monochorionic placentas almost universally contain vascular anastomoses connecting the two fetal circulations: superficial arterio-arterial (AA) and veno-venous (VV) connections, and deep, unidirectional arteriovenous (AV) connections within shared placental cotyledons.

1. Twin-to-Twin Transfusion Syndrome (TTTS)

• Pathophysiology: Affects 10% to 15% of Mo/Di gestations. Unbalanced net blood flow through deep AV anastomoses from the Donor twin to the Recipient twin.
• Donor Twin Manifestations: Severe hypovolemia, decreased renal perfusion, marked oliguria, severe oligohydramnios (Maximum Vertical Pocket [MVP] <2 cm), "stuck twin" appearance (plastered against the uterine wall), fetal growth restriction, and anemia.
• Recipient Twin Manifestations: Severe hypervolemia, increased renal filtration, polyuria, marked polyhydramnios (MVP >8 cm before 20 weeks or >10 cm after 20 weeks), cardiomegaly, systemic hypertension, tricuspid regurgitation, high-output congestive heart failure, and hydrops fetalis.

Quintero Staging System for TTTS

• Definitive Treatment: Fetoscopic Laser Photocoagulation (FLOC / Solomon Technique) between 16 0/7 and 26 0/7 weeks to photocoagulate all communicating vascular anastomoses along the vascular equator, functionally transforming the monochorionic placenta into a dichorionic placenta.

2. Twin Anemia-Polycythemia Sequence (TAPS)

Characterized by slow, chronic feto-fetal transfusion through microscopic AV anastomoses (<1 mm diameter), producing profound hemoglobin discordance (>5 g/dL) without amniotic fluid discordance. Diagnosed by Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV) Doppler: donor twin MCA-PSV >1.5 MoM (severe anemia); recipient twin MCA-PSV <1.0 MoM (severe polycythemia).

3. Twin Reversed Arterial Perfusion (TRAP Sequence / Acardiac Twin)

A rare condition where a normal "pump twin" perfuses an abnormal, non-viable "acardiac twin" retrograde through large AA anastomoses. The pump twin is at high risk of high-output cardiac failure and death (~50%). Managed with radiofrequency ablation (RFA) or bipolar umbilical cord occlusion of the acardiac twin.

4. Monochorionic Monoamniotic (Mo/Mo) Twins

Because both fetuses occupy a single amniotic cavity, umbilical cord entanglement occurs in >95% of cases early in pregnancy. Management requires elective inpatient admission at 24 0/7 to 28 0/7 weeks for daily multi-hour fetal heart rate monitoring, antenatal corticosteroid administration, and planned scheduled cesarean delivery at 32 0/7 to 34 0/7 weeks.

5. Co-Twin Demise in Monochorionic Gestation

If one twin dies in a monochorionic pregnancy, acute maternal-fetal hemodynamics cause sudden, massive reverse exsanguination of the living survivor's blood volume into the lower-resistance vascular bed of the demised twin through patent anastomoses. This causes acute severe hypotension, hypovolemia, ischemic cerebral necrosis (multicystic encephalomalacia), microcephaly, cerebral palsy (~20% to 30%), or acute co-twin demise (~15%). Immediate emergent delivery of the survivor is rarely protective once demise has occurred, as the damage occurs instantaneously at the moment of death.

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Antenatal Surveillance & Delivery Timing Guidelines

ACOG/SMFM Recommended Delivery Timing for Multifetal Gestations

                    [ RECOMMENDED DELIVERY GESTATIONAL AGES ]

  Uncomplicated Di/Di Twins      ─────────────────────────►  38 0/7 – 38 6/7 weeks

  Uncomplicated Mo/Di Twins      ─────────────────────────►  34 0/7 – 37 6/7 weeks
                                                             (typically 36 0/7 – 37 0/7)

  Uncomplicated Mo/Mo Twins      ─────────────────────────►  32 0/7 – 34 0/7 weeks
  (Always via Cesarean Delivery)                             (Inpatient surveillance at 24–28w)

  Triplet Gestation              ─────────────────────────►  35 0/7 – 35 6/7 weeks

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Intrapartum Management of Twin Deliveries

Twin deliveries must take place in an Operating Room (OR) or dedicated surgical delivery suite with full "double-setup" capabilities:

1. Two separate neonatal resuscitation teams and two pre-warmed radiant warmers;
2. Real-time bedside ultrasound machine ready for immediate use;
3. Continuous dual electronic fetal heart rate monitoring;
4. Immediate access to emergency cesarean delivery and obstetric anesthesia.

Mode of Delivery Criteria

• Twin A Vertex / Twin B Vertex: Planned vaginal trial of labor is safe and recommended.
• Twin A Vertex / Twin B Non-Vertex (Breech or Transverse): In pregnancies $\ge 32\text{ weeks}$ with Twin B estimated weight comparable to Twin A (discordance <20%), a vaginal trial of labor is safe and appropriate when managed by an experienced obstetrician skilled in internal podalic version and breech extraction of Twin B.
• Twin A Non-Vertex (Breech or Transverse): Cesarean delivery is recommended for both twins due to the risk of locked twins (Twin A breech chin interlocking with Twin B vertex chin at the pelvic inlet) and cord prolapse.

Specific Intrapartum Nursing Interventions

• Continuous Dual EFM: Apply a fetal scalp electrode (FSE) to Twin A once membranes are ruptured, and monitor Twin B with external ultrasound to prevent signal overlap or artifactual tracing of the same fetus.
• After Delivery of Twin A: Immediately clamp and label the cord ("Twin A"). Do not administer routine oxytocin bolus until Twin B is delivered. Perform immediate real-time ultrasound to confirm Twin B presentation, lie, and heart rate. Maintain abdominal stabilization of Twin B over the pelvic inlet.
• Postpartum Hemorrhage (PPH) Readiness: Marked uterine overdistention dramatically increases the risk of severe postpartum uterine atony. Ensure large-bore IV access, active management of the third stage, high-dose oxytocin infusion, and immediate availability of second-line uterotonics.