Twin Placentas

Slide 1 · Twin placentas

Membranes, shared circulation and clinicopathologic consequences

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Classify twins by chorionicity and amnionicity

Explain membrane anatomy and fetal circulations

Trace the mechanism of twin-to-twin transfusion

Apply findings to prenatal assessment and placental examination

Slide 3 · Three questions define the placentation

Origin

One fertilized ovum or two?

Chorionicity

One shared chorion or two?

Amnionicity

One amniotic cavity or two?

These three classifications answer different questions.

Slide 4 · Two biological routes to twins

Two ova fertilized

Dizygotic twins

One fertilized ovum divides

Monozygotic twins

Slide 5 · Dizygotic: Two fertilizations

Ovum 1 + sperm

Embryo 1

Ovum 2 + sperm

Embryo 2

Dizygotic twins normally form dichorionic diamniotic pregnancies.

Slide 6 · Monozygotic: One conception, two embryos

Single fertilization

Embryonic division

Two embryos

The timing of splitting influences placental and membrane configuration.

Slide 7 · Chorion and amnion are not synonyms

Chorion

Outer fetal membrane

Defines chorionic sharing

Shared chorion permits connected placental circulation

Amnion

Inner fetal membrane

Defines amniotic cavities

Separating membrane may be present or absent

Slide 8 · Placental function: Two circulatory systems

Maternal circulation

Chorionic villus

Fetal circulation

Spiral artery → intervillous space

Umbilical arteries → villi

Umbilical vein → fetus

Exchange occurs across the villous barrier; Maternal and fetal blood normally remain separate.

Slide 9 · The villus is the exchange unit

Maternal intervillous blood

Trophoblast-covered chorionic villus

Fetal villous capillaries

Maternal and fetal circulations are normally separate across an exchange barrier.

Slide 10 · Umbilical vessels supply the fetal placental tree

Two umbilical arteries

Chorionic plate arteries

Villous microcirculation

One umbilical vein

Fetal vessels—not maternal intervillous blood—form intertwin connections.

Slide 11 · Three principal twin placental configurations

DCDA

Two chorions

Two amnions

MCDA

One chorion

Two amnions

MCMA

One chorion

One amnion

Slide 12 · What changes across the three patterns?

Configuration

Chorions

Amniotic sacs

Intertwin membrane

DCDA

Two

Two

Present

MCDA

One

Two

Present

MCMA

One

One

Absent

Slide 13 · DCDA: Two placentas may be separate

Separate dichorionic discs

Each fetus has its own chorion.

Each fetus has its own amniotic sac.

Two placental discs may be distinct.

Slide 14 · DCDA: Two placentas may appear fused

Fused dichorionic discs

Adjacent dichorionic placentas can fuse grossly.

Fusion does not convert two chorions into one.

The intertwin membrane resolves the classification.

Slide 15 · Placental disc count is not chorionicity

What is seen

A single placental mass

Could be monochorionic OR fused dichorionic

What it means

Two separate placental masses

Suggests dichorionicity; Use membrane features and prenatal imaging to confirm

Slide 16 · DCDA intertwin membrane: Four layers

Amnion A

Chorion A

Chorion B

Amnion B

Sac A

Sac B

Schematic membrane sequence between the two amniotic cavities.

Slide 17 · Dichorionic does not establish dizygosity

Dichorionic placenta

Either dizygotic or monozygotic origin

Both pathways may produce DCDA anatomy.

Slide 18 · MCDA: One chorion, two amnions

Shared chorionic placenta

A common chorionic placental unit

Two fluid compartments

A thin intertwin amniotic membrane

Slide 19 · MCDA dividing membrane: Two amnions

Amnion A

Amnion B

Sac A

Sac B

Chorionic layers are absent from the intertwin membrane.

Slide 20 · Why MCDA twins share vascular risk

Twin A circulation

Twin B circulation

Shared placental

Vascular territory

Communicating vessels

Two fetal vascular territories lie within a single chorionic placenta.

Slide 21 · MCMA: One chorion, one amnion

Shared chorionic + amniotic cavity

Common chorionic placenta

A shared amniotic cavity

No intertwin dividing membrane

Slide 22 · No dividing membrane changes the risk profile

Anatomic feature

Both fetuses occupy one amniotic cavity

Two umbilical cords occupy that space

Clinical implication

Cord entanglement is an important concern

Management requires specialized surveillance

Slide 23 · Splitting influences membrane development

Monozygotic twinning

Timing of embryonic separation

Chorionic and amniotic arrangement

Do not infer a precise splitting day from a placental specimen.

Slide 24 · Monozygotic does not mean monochorionic

Monozygotic twins

Can be dichorionic or monochorionic

Chorionicity depends on developmental timing

Dizygotic twins

Usually dichorionic diamniotic

Two ova were independently fertilized

Slide 25 · Anatomical classification: One-view matrix

Pattern

Shared chorion?

Shared amnion?

Vascular linkage?

DCDA

No

No

Not typical; Rare exceptions described

MCDA

Yes

No

Common

MCMA

Yes

Yes

Common

Slide 26 · Examine the entire intertwin septum

Identify membrane insertion

Assess thickness and layers

Correlate with placental mass

Resolve chorionicity

An isolated gross photograph may not capture the full membrane anatomy.

Slide 27 · DCDA membrane is thicker

Amnion

Chorion

Chorion

Amnion

Sac A

Sac B

Two chorionic layers contribute to a thicker septum than MCDA.

Slide 28 · MCDA membrane is comparatively thin

Amnion

Amnion

Sac A

Sac B

Only paired amnions form the dividing membrane.

Slide 29 · Prenatal lambda sign suggests DCDA

Triangular chorionic projection

Lambda / twin-peak insertion

Original schematic — not a diagnostic ultrasound image

Slide 30 · Prenatal T sign suggests MCDA

Thin intertwin membrane meets placenta

T-shaped insertion

Original schematic — not a diagnostic ultrasound image

Slide 31 · Determine chorionicity early

First trimester

Assess chorionicity and amnionicity

Before 13 + 6 weeks

Use multiple sonographic signs

Uncertain

Seek expert review

Slide 32 · Use convergent features, not a single sign

Evaluate the entire intertwin membrane

Inspect its placental insertion (lambda / T sign)

Consider membrane thickness and placental masses

Preserve diagnostic ultrasound images

Slide 33 · Uncertain chorionicity requires escalation

Initial ultrasound uncertain

Targeted transvaginal assessment

Specialist or tertiary review

Do not classify a fused placental mass from disc number alone.

Slide 34 · Surveillance differs by chorionicity

Pregnancy type

Uncomplicated follow-up pattern

DCDA

First-trimester scan; Anatomy scan; Generally every 4 weeks thereafter

Monochorionic

First trimester, then every 2 weeks from 16 weeks

Slide 35 · Why monochorionic surveillance is closer

Shared fetal vascular circuits

Potential hemodynamic imbalance

TTTS may develop

Serial fluid and bladder assessment

Placental anatomy predicts a mechanism that serial ultrasound can monitor.

Slide 36 · Gross assessment begins with identity

Associate each cord with the correct infant

Record separate or fused placental discs

Describe cord insertions and dividing membranes

Preserve orientation before destructive dissection

Slide 37 · Fused placental specimen: Structured examination

Cord A

Cord B

Vascular territories

Mark cord A and cord B

Identify membrane insertion

Map territories on the chorionic plate

Inspect visible crossing vessels

Slide 38 · Membrane sampling confirms architecture

Select membrane insertion

Obtain oriented T section

Examine tissue layers

Correlate with prenatal chorionicity

Correct orientation is important when classifying the intertwin septum.

Slide 39 · Monochorionic placentas can connect two circulations

Twin A circulation

Twin B circulation

Shared placental

Vascular territory

Communicating vessels

Anastomoses connect fetal vessels across placental territories.

Slide 40 · Vascular communications have different patterns

Artery–artery

Direct arterial connection

Vein–vein

Direct venous connection

Artery–vein

Deep unidirectional-type pathway

The number, caliber and balance of communications matter; Schematic classes.

Slide 41 · Net flow—not the mere connection—drives TTTS

Twin A circulation

Twin B circulation

Shared placental

Vascular territory

Preferential net transfer

Illustrated preferential net transfer between fetal circulations.

Slide 42 · TTTS: Defining pathophysiology

Monochorionic vascular anastomoses

Preferential net blood transfer

Donor underperfusion

Recipient volume overload

Both fetuses can be affected by one shared placental vascular imbalance.

Slide 43 · The donor is relatively underperfused

Hemodynamic change

Net blood transfer away from donor

Reduced effective circulating volume

Consequences

Reduced urine production

Reduced amniotic fluid volume

Slide 44 · The recipient becomes fluid overloaded

Hemodynamic change

Net blood transfer toward recipient

Increased effective circulating volume

Consequences

Increased urine and amniotic fluid

Cardiovascular strain in severe disease

Slide 45 · Two fetuses, opposing fluid phenotypes

Donor

Underperfused

Oligohydramnios

Bladder may become difficult to visualize

Recipient

Volume overloaded

Polyhydramnios

Cardiovascular effects may develop

Slide 46 · Ultrasound diagnosis requires fluid discordance

Assessment

Traditional diagnostic threshold

Donor deepest vertical pocket

≤ 2 cm

Recipient deepest vertical pocket

≥ 8 cm

Important nuance

Some guidance uses ≥ 10 cm after 20 weeks

Slide 47 · Quintero stages describe observed severity

I

Oligohydramnios–polyhydramnios

II

Donor bladder not visible

III

Critical fetal Doppler abnormality

IV

Hydrops in at least one twin

V

Fetal death

Slide 48 · Stage I: Fluid imbalance with visible donor bladder

Prerequisite

Donor oligohydramnios

Recipient polyhydramnios

Discriminator

Donor bladder remains visible

Severity cannot be judged from fluid alone

Slide 49 · Stage II: Donor bladder is not visualized

TTTS fluid discordance

Donor bladder not seen on scan

Quintero stage II

Bladder assessment is part of routine monochorionic surveillance.

Slide 50 · Stage III: Critical Doppler abnormality

Absent or reversed umbilical artery end-diastolic flow

Reversed ductus venosus flow

Pulsatile umbilical venous flow

An abnormality in either fetus may satisfy this stage

Slide 51 · Stages IV–V: Hydrops or fetal demise

Stage IV

Hydrops in one or both twins

Represents severe fetal circulatory decompensation

Stage V

Death of one or both twins

A possible severe outcome of transfusion imbalance

Slide 52 · Quintero stage is not a time sequence

Observed ultrasound findings

Staging label

Clinical decision support

Progression need not pass through each stage in numerical order.

Slide 53 · Severe TTTS may endanger both fetuses

Persistent preferential shunting

Donor hypoperfusion + recipient overload

Possible death of one or both twins

The severity derives from asymmetric circulatory consequences.

Slide 54 · Laser therapy targets the shared channels

Twin A circulation

Twin B circulation

Shared placental

Vascular territory

Ablation line

Fetoscopic laser coagulation aims to interrupt placental vascular connections.

Slide 55 · SMFM treatment context: Stages II–IV

For stages II–IV presenting at 16–26 weeks, fetoscopic laser is standard treatment

Eligible cases should be evaluated at a fetal intervention center

Stage I often requires close surveillance; Treatment is individualized

Slide 56 · Vascular territory need not equal fetal share

Cord A

Cord B

Vascular territories

Identify the vascular equator

Record relative placental territory for each cord

Describe anastomoses where visible

Correlate with growth discordance

Slide 57 · Pathology report: Information for each twin

Chorionicity and amnionicity

Individual cord and membrane findings

Separate diagnoses attributable to twin A and twin B

Vascular territory and communications in monochorionic placentas

Slide 58 · Injection studies map intertwin vessels

Fresh monochorionic placenta

Cannulate fetal vessels

Inject distinct dyes

Map vascular interconnections

Specialized studies can delineate communications beyond routine inspection.

Slide 59 · Twin discordance has multiple causes

Growth

Twin size can differ

Development

Karyotype or malformations may differ

Placental pathology

Territory and lesions may differ

Discordance is not synonymous with TTTS.

Slide 60 · TTTS versus selective growth restriction

TTTS

Mechanism centers on intertwin hemodynamic imbalance

Characteristically opposing fluid volumes

Selective fetal growth restriction

Mechanism commonly includes unequal placental sharing

Abnormal growth does not itself establish TTTS

Slide 61 · TTTS versus TAPS

TTTS

Amniotic fluid imbalance defines classic TTTS

Oligohydramnios–polyhydramnios pattern

TAPS

Chronic intertwin hematologic discordance

May lack classic fluid imbalance; Doppler-based assessment

Slide 62 · Case: One mass with a thick membrane

Twins have a single gross placental mass, but the dividing membrane contains chorionic layers. What is the chorionicity?

Dichorionic. Fused DCDA placentas can resemble one disc grossly.

The disc count alone is misleading.

The membrane layers resolve the classification.

Slide 63 · Case: Thin membrane and two fluid sacs

Two fetuses occupy separate amniotic sacs within one chorion; The dividing membrane has two amnions only. Classify the placenta.

Monochorionic diamniotic (MCDA).

Two sacs do not imply two chorions.

Shared placental vasculature is clinically important.

Slide 64 · Case: Shared sac with two cords

A monochorionic placenta has no true intertwin membrane. Which configuration is present?

Monochorionic monoamniotic (MCMA).

Both fetuses occupy a common amniotic cavity.

Cord entanglement is an important additional risk.

Slide 65 · Case: Opposing deepest fluid pockets

A monochorionic diamniotic pair has donor DVP 1.5 cm and recipient DVP 9 cm at 19 weeks. Which syndrome should be suspected?

Twin-to-twin transfusion syndrome (TTTS).

The values satisfy the traditional fluid criteria.

Evaluate donor bladder and Doppler for staging.

Slide 66 · Case: Absent donor bladder

A TTTS pregnancy shows fluid discordance and the donor bladder is not visualized; No more advanced criteria are described. What stage fits?

Quintero stage II.

Absent donor bladder is the defining stage-II finding.

Doppler assessment remains essential.

Slide 67 · Interpretation workflow: From pattern to risk

Assign chorionicity + amnionicity

Characterize membrane and placental anatomy

Look for fetal vascular connections

Interpret discordance and complications

Do not diagnose TTTS solely from twin size or a shared placental mass.

Slide 68 · Rapid comparison: Which clue matters most?

Observation

Most informative inference

Thick septum with chorion

Dichorionic membranes

Thin two-amnion septum

Monochorionic diamniotic

No dividing septum in shared chorion

Monochorionic monoamniotic

Paired fluid imbalance in MCDA

Evaluate TTTS and Quintero stage

Slide 69 · Four points to retain

Zygosity, chorionicity and amnionicity describe different features

Dichorionic discs can be fused; Disc number alone is insufficient

Monochorionic placentas connect fetal circulations

Preferential shunting can underperfuse a donor and overload a recipient

Slide 70 · Primary reference

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026).

Chapter 22: The Female Genital Tract. Gestational and Placental Disorders; Twin Placentas, p. 936.

Original diagrams throughout this lecture are editable schematics, not reproduced textbook figures.

Slide 71 · Supplementary references

Khalil A, et al. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol. 2025;65:253–276. doi:10.1002/uog.29166.

SMFM Consult Series #72. Twin-twin transfusion syndrome and twin anemia–polycythemia sequence. AJOG. 2024. doi:10.1016/j.ajog.2024.07.017.

Fitzgerald B. Histopathological examination of the placenta in twin pregnancies. APMIS. 2018;126:626–637. doi:10.1111/apm.12829.

RCPA Macroscopic Cut-Up Manual: Placenta—multiple gestation.