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Summary Table

Feature	Detail
Risk Factors	Advanced maternal age, assisted reproductive technology (ART), family history of dizygotic twins (maternal side), high parity, African descent, prior multiple gestation
Etiology	Dizygotic: fertilization of two or more separate ova. Monozygotic: splitting of a single fertilized ovum at various time points post-fertilization
Patient Presentation	Uterine size greater than expected for gestational age, excessive nausea/vomiting in early pregnancy, rapid weight gain, elevated beta-hCG higher than expected for dates
Classic Physical Exam	Fundal height larger than dates, multiple fetal poles on Leopold maneuvers, auscultation of more than one distinct fetal heart rate
Key Diagnostic Results	Ultrasound confirming number of fetuses, chorionicity, and amnionicity. Best determined in the first trimester. Lambda sign (dichorionic) vs. T-sign (monochorionic)
Management	Increased surveillance frequency, serial cervical length measurement, growth ultrasounds every 3 to 4 weeks, timing of delivery based on chorionicity and amnionicity, corticosteroids for anticipated preterm birth
Keywords	"Uterus large for dates," "twin peak sign," "lambda sign," "T-sign," "discordant growth," "twin-to-twin transfusion syndrome," "stuck twin"

1. Pathophysiology

The foundation of understanding multiple gestation lies in when the zygote splits (for monozygotic twins) and how many placentas and amniotic sacs are present, because this determines the entire risk profile.

Dizygotic (fraternal) twins arise from the release and fertilization of two separate ova by two separate sperm. Each embryo implants independently and develops its own placenta, chorion, and amnion. These twins are always dichorionic-diamniotic (DCDA) and carry the lowest risk profile among all twin types. Genetically, they are no more similar than any two siblings. The tendency toward dizygotic twinning is heritable through the maternal line and is linked to higher baseline FSH levels, which promote multiple follicle development.

Monozygotic (identical) twins result from a single fertilized ovum that splits after conception. The critical variable is the timing of division, which determines the membrane configuration:

Division at day 0 to 3 (before morula stage): two separate embryos implant independently, each with its own placenta, chorion, and amnion. This produces dichorionic-diamniotic twins, indistinguishable from dizygotic twins on imaging alone.
Division at day 4 to 8 (after trophoblast differentiation but before amnion formation): the embryos share one placenta and one chorion but develop separate amniotic cavities. This is the monochorionic-diamniotic (MCDA) configuration and accounts for roughly 70% of monozygotic twins.
Division at day 8 to 12 (after amnion formation): the embryos share one placenta, one chorion, and one amniotic cavity. This monochorionic-monoamniotic (MCMA) configuration carries the highest risk due to cord entanglement.
Division after day 13: incomplete separation results in conjoined twins.

The reason monochorionic twins carry disproportionately higher morbidity is the shared placental circulation. Vascular anastomoses between the two fetal circulations are nearly universal in monochorionic placentas. When these anastomoses become unbalanced, one twin becomes a net donor (losing blood volume) while the other becomes a net recipient (receiving excess volume). This is the basis of twin-to-twin transfusion syndrome (TTTS), the single most dangerous complication unique to monochorionic twins.

All multiple gestations share a common pathway toward complications: the uterus is overdistended, placental demand is increased, and maternal cardiovascular adaptation is stressed beyond what a singleton pregnancy requires. This explains the higher incidence of preterm labor, preeclampsia, gestational diabetes, placenta previa, postpartum hemorrhage, and intrauterine growth restriction in multiples.

2. Classification and Clinical Manifestation

Zygosity and Chorionicity

Timing of Split	Chorionicity / Amnionicity	Frequency (among monozygotic)	Ultrasound Sign	Risk Level
N/A (dizygotic)	Dichorionic-Diamniotic (DCDA)	All dizygotic twins	Lambda sign (twin peak sign): triangular projection of placental tissue between membranes	Lowest
Day 0 to 3	Dichorionic-Diamniotic (DCDA)	~30%	Lambda sign	Lowest
Day 4 to 8	Monochorionic-Diamniotic (MCDA)	~68%	T-sign: thin membrane inserts directly onto placenta without intervening tissue	High
Day 8 to 12	Monochorionic-Monoamniotic (MCMA)	~1 to 2%	No dividing membrane visible	Highest
After day 13	Conjoined	Rare	Fetal bodies that do not separate	Highest

Complications

Complication	DCDA	MCDA	MCMA
Preterm birth	Increased	Further increased	Highest risk
Preeclampsia	Increased	Increased	Increased
Twin-to-twin transfusion syndrome (TTTS)	Not applicable	10 to 15%	Can occur
Twin anemia-polycythemia sequence (TAPS)	Not applicable	3 to 5%	Can occur
Selective intrauterine growth restriction	Possible	More common	Possible
Cord entanglement	Not applicable	Not applicable	Major risk
Twin reversed arterial perfusion (TRAP)	Not applicable	Rare	Rare
Single fetal demise with co-twin risk	Low risk to surviving twin	High risk: shared circulation can cause acute hypotension, ischemic injury, or death of co-twin	High risk

Twin-to-Twin Transfusion Syndrome (Quintero Staging)

Stage	Criteria
I	Polyhydramnios in recipient (deepest vertical pocket greater than 8 cm) and oligohydramnios in donor (deepest vertical pocket less than 2 cm), donor bladder still visible
II	Donor bladder not visible on ultrasound (absent filling)
III	Critically abnormal Doppler studies: absent or reversed end-diastolic flow in the umbilical artery of the donor, or reversed flow in the ductus venosus or pulsatile flow in the umbilical vein of the recipient
IV	Hydrops fetalis in one or both twins
V	Demise of one or both twins

3. Diagnostic Workup

Test	Purpose	Timing
First-trimester ultrasound (best initial test and most accurate for chorionicity)	Determine number of fetuses, chorionicity, amnionicity	11 to 14 weeks (ideally before 14 weeks)
Lambda sign vs. T-sign	Distinguish dichorionic from monochorionic placentation	First trimester
Nuchal translucency	Aneuploidy screening, also screens for TTTS risk if discordant	11 to 14 weeks
Serial growth ultrasound	Monitor for discordant growth, TTTS, selective growth restriction	Every 3 to 4 weeks starting at 16 weeks for MCDA; every 4 weeks for DCDA
Cervical length measurement	Screen for preterm birth risk	Starting at 16 weeks, serially
Middle cerebral artery peak systolic velocity (MCA-PSV) Doppler	Detect fetal anemia (TAPS, post single fetal demise)	As indicated in monochorionic twins
Umbilical artery Doppler	Assess placental resistance, especially in growth-restricted twins	As indicated
Maternal serum screening	Note: multiple gestation alters normal reference values for analytes	Adjusted interpretation required

First-trimester ultrasound is the single most important diagnostic step. Determining chorionicity is best done before 14 weeks, when the lambda sign and T-sign are most reliable. After 14 weeks, the inter-twin membrane thins and these signs become less distinct, making chorionicity assignment unreliable. This is a commonly tested concept: the exam will present a scenario where a woman with twins had no early ultrasound and now presents at 20-plus weeks, and you are asked whether chorionicity can still be determined accurately. The answer is that it becomes progressively difficult and unreliable.

The lambda sign (also called the twin peak sign) represents a triangular wedge of placental tissue extending into the base of the inter-twin membrane, indicating that each twin has its own chorion with placental tissue between them. This confirms dichorionic placentation. The T-sign shows the membrane meeting the placental surface at a right angle with no intervening tissue, confirming monochorionic placentation.

For MCDA twins, surveillance every 2 weeks starting from 16 weeks is recommended to screen for TTTS. The hallmark finding is the polyhydramnios-oligohydramnios sequence: the recipient twin has a deepest vertical pocket greater than 8 cm while the donor has less than 2 cm. The donor twin may appear "stuck" against the uterine wall due to severe oligohydramnios with limited movement, hence the term "stuck twin."

Discordant growth is defined as a difference of 20% or more between estimated fetal weights, calculated as: the weight of the larger twin minus the weight of the smaller twin, divided by the weight of the larger twin. This finding warrants closer surveillance and Doppler assessment.

4. Management and Treatment

Clinical Scenario	Management
All multiple gestations	Increased caloric intake (additional 300 kcal/day per fetus), iron and folate supplementation, serial growth monitoring
DCDA twins, uncomplicated	Delivery at 37 to 38 weeks
MCDA twins, uncomplicated	Delivery at 36 weeks
MCMA twins, uncomplicated	Delivery at 32 to 34 weeks with antenatal corticosteroids; consider inpatient monitoring from 24 to 28 weeks onward
Triplets (trichorionic)	Delivery at 35 to 36 weeks
Triplets (dichorionic or monochorionic component)	Delivery at 34 to 35 weeks
TTTS (Quintero Stage I)	Close surveillance every 1 to 2 weeks; intervention if progression occurs
TTTS (Quintero Stage II to IV)	Fetoscopic laser photocoagulation of placental anastomoses (treatment of choice)
Selective fetal growth restriction in MCDA	Doppler surveillance; if critically abnormal, consider laser ablation or selective cord occlusion
TRAP sequence	Radiofrequency ablation of the acardiac twin's cord
Preterm labor in multiples	Betamethasone 12 mg intramuscular, two doses 24 hours apart (if between 24 and 34 weeks); magnesium sulfate for neuroprotection if less than 32 weeks
Preeclampsia prevention	Low-dose aspirin (150 mg daily) starting at 12 to 16 weeks, as multiple gestation is a risk factor
Mode of delivery	Vaginal delivery acceptable if the presenting twin (Twin A) is vertex. If Twin A is non-vertex, cesarean delivery is preferred

Antenatal corticosteroids follow the same dosing protocol as in singleton pregnancy: betamethasone 12 mg intramuscular every 24 hours for two doses, or dexamethasone 6 mg intramuscular every 12 hours for four doses. The threshold for administration is anticipation of delivery between 24 and 34 weeks of gestation.

Fetoscopic laser photocoagulation is the definitive treatment for TTTS at Stage II and above. The procedure uses a fetoscope inserted into the recipient twin's amniotic cavity to identify and ablate the communicating placental vessels. This interrupts the unbalanced transfusion and has been shown to improve survival and neurodevelopmental outcomes compared to serial amnioreduction, which was the prior standard. Serial amnioreduction (removing excess fluid from the recipient sac) is now considered a temporizing measure or an option when laser therapy is unavailable.

For MCMA twins, the primary concern is cord entanglement, which can cause sudden fetal demise. These pregnancies often require inpatient monitoring starting from viability (24 to 28 weeks) with continuous or near-continuous fetal heart rate surveillance. Delivery is planned early, at 32 to 34 weeks, to avoid the unpredictable risk of cord accident.

Regarding mode of delivery, the key exam-tested principle is the presentation of Twin A (the presenting twin closest to the cervix). If Twin A is vertex, vaginal delivery can be attempted regardless of Twin B's presentation, because Twin B can be delivered by internal podalic version and breech extraction after Twin A is delivered. If Twin A is non-vertex, cesarean section is indicated. For MCMA twins, cesarean delivery is generally recommended regardless of presentation due to the risk of cord entanglement during labor.

Tocolysis is not recommended for the sole purpose of prolonging a multiple gestation to term; the goal is to achieve a 48-hour window for corticosteroid administration. Progesterone supplementation for preterm birth prevention, which is used in singleton pregnancies with a short cervix, has not been shown to be effective in multiple gestations.

5. Differential Diagnosis and Distractors

Differential Diagnosis	Why It Is Similar	Key Discriminator
Molar pregnancy (complete or partial)	Also presents with uterus larger than dates, elevated beta-hCG, and excessive nausea	Ultrasound shows "snowstorm" pattern or grape-like vesicles instead of viable fetuses; beta-hCG is often dramatically elevated beyond what even twins produce
Polyhydramnios (singleton)	Uterus large for dates, similar to multiple gestation on initial exam	Ultrasound reveals only one fetus with excess amniotic fluid; workup targets causes like fetal anomaly, gestational diabetes, or fetal anemia
Macrosomia	Uterus large for dates	Single fetus with estimated weight above the 90th percentile; associated with maternal diabetes or constitutional factors
Incorrect dating	Uterus "large for dates" when the true gestational age is actually further along	Crown-rump length on early ultrasound resolves the discrepancy; no additional fetuses seen
TTTS vs. selective intrauterine growth restriction	Both show discordant twin sizes with abnormal fluid volumes	In TTTS, the smaller twin (donor) has oligohydramnios with a non-visible bladder, and the larger twin (recipient) has polyhydramnios. In selective growth restriction, the smaller twin may have normal to reduced fluid but the bladder remains visible, and Doppler abnormalities in the umbilical artery are the primary finding
TTTS vs. twin anemia-polycythemia sequence (TAPS)	Both involve unbalanced blood flow between monochorionic twins	TAPS shows a large difference in MCA-PSV Doppler between the twins (one anemic, one polycythemic) without the polyhydramnios-oligohydramnios sequence that defines TTTS. Fluid volumes are near-normal in TAPS

6. Traps and High-Yield Pearls

The most common way students lose points on multiple gestation questions is by failing to determine chorionicity before selecting a management plan. The exam will often present twins without telling you the membrane status outright, and students jump to generic twin management. The correct first step is always to establish whether the twins are dichorionic or monochorionic, because the entire surveillance strategy and delivery timing depend on this distinction. If the vignette does not provide this information, the next best step is first-trimester ultrasound to assess chorionicity.

A second frequent trap involves TTTS recognition. The vignette will describe a monochorionic twin pregnancy at around 20 weeks with one twin surrounded by abundant fluid and the other twin appearing immobile and compressed against the uterine wall ("stuck twin"). Students who do not recognize the polyhydramnios-oligohydramnios sequence may incorrectly attribute the findings to simple growth discordance or preterm labor. The tested concept is identifying TTTS and knowing that fetoscopic laser ablation is the treatment of choice for Stage II and above.

A third trap is the delivery timing question. Exam writers will present an uncomplicated DCDA twin pregnancy and ask when to deliver. Students who recall that "twins deliver early" may select 34 or 35 weeks, which is too early for uncomplicated dichorionic twins (correct answer: 37 to 38 weeks). Conversely, for MCMA twins, waiting beyond 32 to 34 weeks is too late given the cord entanglement risk. Memorizing the delivery thresholds by chorionicity and amnionicity type is essential.

Finally, remember that progesterone supplementation does not reduce preterm birth risk in multiple gestations, despite its efficacy in singleton pregnancies with a short cervix. This is a well-established distractor on exam questions about preterm birth prevention in twins.

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