Gemelli

01Pathophysiology

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.

02Classification and Clinical Manifestation

Zygosity and Chorionicity

Complications

Twin-to-Twin Transfusion Syndrome (Quintero Staging)

03Diagnostic Workup

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.

04Management and Treatment

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.

05Differential Diagnosis and Distractors

06Traps 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.