# Multiple Gestation: Chorionicity, Surveillance, and Delivery

## Epidemiology

Twin pregnancies account for approximately 3% of all births in the United States, a rate that has increased significantly with the use of assisted reproductive technologies. The spontaneous twinning rate is roughly 1 in 80 pregnancies. Higher-order multiples have decreased with improved ART practices, particularly the adoption of elective single embryo transfer. Dizygotic (fraternal) twins make up about 70% of all twin pregnancies and are always dichorionic-diamniotic (DCDA) because they arise from separate fertilization events. Monozygotic (identical) twins account for the remaining 30%, and their chorionicity depends on when the embryo splits.

## Determining Chorionicity and Amnionicity

### Timing of Monozygotic Twin Splitting

The timing of embryo splitting determines the placental and membrane configuration. Splitting within the first 3 days (before the morula stage) produces dichorionic-diamniotic twins, accounting for 20 to 30% of monozygotic twins. Splitting between days 4 and 8 (at the blastocyst stage, with division of the inner cell mass) produces monochorionic-diamniotic (MCDA) twins, the most common configuration at 70 to 75%. Splitting between days 8 and 12 (after amnion formation) produces monochorionic-monoamniotic (MCMA) twins, which occur in only 1 to 2% of cases. Splitting after day 13 results in conjoined twins, an extremely rare occurrence.

### Ultrasound Determination (Best in First Trimester)

Chorionicity determination in the first trimester is the most critical step in managing any twin pregnancy because it dictates the entire surveillance plan. Dichorionic-diamniotic twins show either two separate placentas or fused placentas with the "twin peak" (lambda) sign -- a triangular wedge of placental tissue extending between the membrane layers at the placental insertion site -- along with a thick intertwin membrane. Monochorionic-diamniotic twins show a single placenta with a thin intertwin membrane, the same fetal sex, and no twin peak sign. Instead, the membrane inserts directly into the placenta, creating the "T-sign." Monochorionic-monoamniotic twins share a single placenta with no dividing membrane, and cord entanglement is typically visible. The first trimester is the most accurate time for making this determination, and when chorionicity is uncertain, the pregnancy should be managed as monochorionic (the higher-risk category).

<image>Ultrasound images comparing the twin peak (lambda) sign in dichorionic twins with the T-sign in monochorionic diamniotic twins, with labeled placental tissue, membrane layers, and insertion points</image>

## Risks by Chorionicity

### All Twins

All twin pregnancies carry increased risks compared to singletons, including a 50% rate of delivery before 37 weeks, a 2- to 3-fold higher risk of preeclampsia, increased rates of gestational diabetes, anemia, postpartum hemorrhage (from uterine overdistension), and cesarean delivery.

### Dichorionic Twins

Dichorionic twins have separate placentas and independent circulations, making them lower risk than monochorionic twins. Growth discordance can occur and is defined as a greater than 20% difference in estimated fetal weight, which warrants increased surveillance.

### Monochorionic Twins -- Unique Complications

Monochorionic twins share a single placenta with vascular anastomoses (arteriovenous, arterioarterial, and venovenous connections), which create risks for several unique and potentially devastating complications. Twin-to-twin transfusion syndrome (TTTS) affects 10 to 15% of MCDA twins. Twin anemia-polycythemia sequence (TAPS) involves chronic imbalanced blood flow without the amniotic fluid derangements seen in TTTS. Selective intrauterine growth restriction (sIUGR) occurs when one twin's placental share is inadequate. Twin reversed arterial perfusion (TRAP) sequence involves an acardiac twin perfused by its co-twin. When single fetal demise occurs in a monochorionic pregnancy, the surviving co-twin faces a 15 to 20% risk of death or neurologic injury through acute hypotension transmitted via the vascular anastomoses.

### Monoamniotic Twins

Monoamniotic twins face the additional risk of cord entanglement, which is virtually universal. The risk of fetal death from cord compression is substantial, and inpatient monitoring typically begins at 24 to 28 weeks with continuous or frequent fetal heart rate monitoring.

## Twin-to-Twin Transfusion Syndrome (TTTS)

### Pathophysiology

TTTS results from unbalanced blood flow through placental arteriovenous anastomoses. The donor twin becomes hypovolemic, anemic, and oliguric, developing oligohydramnios and becoming the "stuck twin." The recipient twin becomes hypervolemic, polycythemic, and polyuric, with polyhydramnios. Without treatment, TTTS can progress rapidly to hydrops and death of one or both twins.

### Quintero Staging

TTTS is staged by the Quintero system. Stage I shows polyhydramnios in the recipient (deepest vertical pocket above 8 cm) and oligohydramnios in the donor (deepest vertical pocket below 2 cm), but the donor's bladder is still visible. In Stage II, the donor's bladder is no longer visible, indicating anuria. Stage III features critically abnormal Doppler findings, including absent or reversed end-diastolic flow in the umbilical artery or abnormal ductus venosus or umbilical vein pulsations. Stage IV shows hydrops in one or both twins. Stage V indicates death of one or both twins.

| Quintero Stage | Findings | Management |
|---|---|---|
| I | Polyhydramnios/oligohydramnios; donor bladder visible | Surveillance q1-2 weeks; may regress |
| II | Donor bladder not visible (anuria) | Fetoscopic laser photocoagulation |
| III | Critically abnormal Dopplers (AEDF/REDF, abnormal DV) | Fetoscopic laser photocoagulation |
| IV | Hydrops in one or both twins | Fetoscopic laser photocoagulation |
| V | Death of one or both twins | — |

### Management

Stage I TTTS may be managed with close surveillance using serial ultrasound every 1 to 2 weeks, as some cases remain stable or regress. Stages II through IV are treated with fetoscopic laser photocoagulation of the placental anastomoses, the gold standard intervention performed at specialized centers. Laser treatment achieves survival of at least one twin in 85 to 90% of cases and survival of both twins in 60 to 70%, though complications including PPROM, preterm delivery, and chorioamnionitis can occur. Serial amnioreduction (drainage of polyhydramnios) is a palliative measure that is inferior to laser for advanced stages. Selective reduction by cord occlusion is considered when one twin is previable or has lethal abnormalities.

<image>Diagram of twin-to-twin transfusion syndrome pathophysiology showing a shared monochorionic placenta with arteriovenous anastomoses, the donor twin with oligohydramnios and shrunken bladder, and the recipient twin with polyhydramnios, distended bladder, and potential hydrops</image>

## Selective Intrauterine Growth Restriction (sIUGR) in MC Twins

Selective IUGR is diagnosed when the estimated fetal weight of one twin falls below the 10th percentile. It is classified by umbilical artery Doppler patterns. Type I, with positive end-diastolic flow, carries a generally favorable prognosis and is managed with surveillance. Type II, with persistently absent or reversed end-diastolic flow, carries higher risk of intrauterine fetal death and neurologic injury to the co-twin. Type III shows intermittent absent or reversed end-diastolic flow with a bidirectional pattern and an unpredictable clinical course. Management options depend on the type, gestational age, and severity, and include continued surveillance, laser treatment, cord occlusion of the growth-restricted twin, or delivery.

## Fetal Surveillance Protocols

### Dichorionic Twins

Dichorionic twins undergo an anatomy survey at 18 to 22 weeks, growth ultrasound every 4 weeks starting at 24 to 28 weeks, and antenatal testing (NST or BPP) beginning at 36 weeks (earlier if complications develop). Standard screening for preeclampsia and gestational diabetes is performed.

### Monochorionic-Diamniotic Twins

MCDA twins require more intensive surveillance. After first-trimester dating and chorionicity confirmation, ultrasound is performed every 2 weeks from 16 through 28 weeks to screen for TTTS, assessing amniotic fluid volumes (deepest vertical pocket), bladder visualization in each twin, and Doppler studies. Growth ultrasound is performed every 2 to 4 weeks from 24 to 28 weeks onward. Antenatal testing begins at 32 weeks and is performed twice weekly. Fetal echocardiography at 18 to 22 weeks is indicated because of the increased risk of cardiac anomalies in monochorionic twins.

### Monoamniotic Twins

Monoamniotic twins require the most intensive monitoring. They are typically admitted to the hospital at 24 to 28 weeks for daily or continuous fetal heart rate monitoring. Growth ultrasound is performed every 2 to 3 weeks.

## Delivery Timing

### Evidence-Based Recommendations (ACOG/SMFM)

For uncomplicated DCDA twins, delivery is recommended at 38 weeks 0 days to 38 weeks 6 days. For uncomplicated MCDA twins, delivery is recommended at 36 weeks 0 days to 37 weeks 6 days (34 to 36 weeks if the pregnancy was complicated by prior TTTS or laser treatment). For MCMA twins, delivery is recommended at 32 weeks 0 days to 34 weeks 0 days by cesarean delivery. Earlier delivery is indicated for any complication including preeclampsia, IUGR, abnormal Dopplers, TTTS, or fetal distress.

| Chorionicity | Recommended Delivery Timing | Mode |
|---|---|---|
| DCDA (uncomplicated) | 38+0 to 38+6 weeks | Vaginal if presenting twin vertex |
| MCDA (uncomplicated) | 36+0 to 37+6 weeks | Vaginal if presenting twin vertex |
| MCDA (prior TTTS/laser) | 34+0 to 36+0 weeks | Individualized |
| MCMA | 32+0 to 34+0 weeks | Cesarean delivery |

### Mode of Delivery

When the presenting twin is vertex, vaginal delivery is appropriate. If the second twin is non-vertex after delivery of the first, internal podalic version and breech extraction is an option for experienced providers. When the presenting twin is non-vertex, cesarean delivery is recommended. Monoamniotic twins are delivered by cesarean because of the risk of cord entanglement during labor. Continuous monitoring of both twins is required during labor, and the capability for immediate cesarean delivery for second-twin emergencies must be available.

## Clinical Pearls

Chorionicity determination in the first trimester is the single most important step in managing twin pregnancies. It drives the entire surveillance plan and cannot be reliably determined later in gestation.

Monochorionic twins require ultrasound every 2 weeks from 16 weeks onward to screen for TTTS. Dichorionic twins do not need this intensive surveillance.

The "stuck twin" -- with oligohydramnios and a non-visible bladder in an MCDA pregnancy -- should immediately raise suspicion for twin-to-twin transfusion syndrome.

When a single fetal demise occurs in a monochorionic pregnancy, the surviving co-twin faces acute risk through the shared vascular anastomoses. Urgent maternal-fetal medicine consultation and MRI of the surviving twin's brain at 4 to 6 weeks are indicated to assess for ischemic injury.

Progesterone supplementation and cerclage for short cervix should not be offered to twin gestations. Available evidence does not support these interventions in multiple pregnancies and cerclage may worsen outcomes.

Twin pregnancies should not continue beyond 38 weeks for DCDA twins or 37 weeks for MCDA twins, even when uncomplicated.

## References

- ACOG Practice Bulletin No. 231: Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies (2021)
- SMFM Consult Series No. 46: Twin-Twin Transfusion Syndrome (2019)
- Senat MV et al. Endoscopic laser surgery versus serial amnioreduction for severe twin-to-twin transfusion syndrome. N Engl J Med. 2004;351:136-144
- Khalil A et al. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol. 2016;47:247-263
- Cheong-See F et al. Prospective risk of stillbirth and neonatal complications in twin pregnancies: systematic review and meta-analysis. BMJ. 2016;354:i4353