Reproductive · Obstetrics
The facts most likely to be tested
Dizygotic twins result from the fertilization of two separate ova by two separate sperm and are always dichorionic-diamniotic.
Monozygotic twins resulting from division within the first 3 days post-fertilization are dichorionic-diamniotic, while division between days 4-8 results in monochorionic-diamniotic twins.
Monochorionic-monoamniotic twins arise from division between days 9-12 and carry the highest risk of cord entanglement and fetal demise.
Twin-twin transfusion syndrome (TTTS) occurs exclusively in monochorionic gestations due to unbalanced vascular anastomoses in the shared placenta.
Ultrasound performed in the first trimester (10-14 weeks) is the most accurate method for determining chorionicity via the lambda sign or T-sign.
Monochorionic pregnancies require serial ultrasound surveillance every 2 weeks starting at 16 weeks to screen for TTTS.
Multiple gestation significantly increases the risk of preterm labor, preeclampsia, and gestational diabetes, necessitating increased frequency of prenatal visits.
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A 28-year-old G2P1 woman at 20 weeks gestation presents for a routine anatomy scan. Ultrasound reveals two fetuses with a single placenta and a thin dividing membrane meeting at the uterine wall at a 90-degree angle, known as the T-sign. The larger fetus shows polyhydramnios, while the smaller fetus shows oligohydramnios and a distended bladder. The patient has no significant past medical history.
What is the most likely diagnosis and the underlying pathophysiology?
Twin-twin transfusion syndrome (TTTS) due to placental vascular anastomoses.
The presence of the T-sign confirms a monochorionic gestation, which is a prerequisite for TTTS. The clinical findings of discordant amniotic fluid volumes and bladder size are classic for TTTS resulting from unbalanced blood flow through shared placental vessels.
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Etiology / Epidemiology
Incidence driven by advanced maternal age and assisted reproductive technology (ART). Dizygotic twins result from multiple ovulations; monozygotic from single zygote cleavage.
Clinical Manifestations
Excessive uterine size > dates and hyperemesis gravidarum. Palpation of multiple fetal poles on physical exam.
Diagnosis
Transvaginal ultrasound is the gold standard. Determine chorionicity by 10-14 weeks via lambda sign or T-sign.
Treatment
Serial ultrasound surveillance for growth discordance. Avoid preterm labor; elective delivery at 37-38 weeks for uncomplicated dichorionic twins.
Prognosis
High risk of preterm birth and low birth weight. Monochorionic gestations carry risk of twin-twin transfusion syndrome.
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Epidemiology & Etiology
Rates are rising due to ART and delayed childbearing. Dizygotic (fraternal) twinning is influenced by genetics and maternal age, while monozygotic (identical) twinning is a random event.
Pertinent Anatomy
Chorionicity determines risk: Dichorionic/Diamniotic (separate placentas) is safest. Monochorionic (shared placenta) risks vascular anastomoses, while Monoamniotic (shared sac) risks cord entanglement.
Pathophysiology
Rapid uterine expansion leads to increased mechanical stretch, triggering early cervical ripening. Shared placental circulation in monochorionic twins causes hemodynamic imbalance, potentially leading to twin-twin transfusion syndrome (TTTS) where one twin becomes a donor (hypovolemic) and one a recipient (hypervolemic).
Clinical Manifestations
Patients present with fundal height > gestational age by >3cm. Expect hyperemesis gravidarum due to elevated hCG levels. Red flags include vaginal bleeding or contractions indicating preterm labor.
Diagnosis
Transvaginal ultrasound is the definitive diagnostic tool. The lambda sign (or twin peak sign) confirms dichorionicity, while the T-sign indicates monochorionicity. Early dating is critical for accurate growth discordance monitoring.
Treatment
Management focuses on serial ultrasounds every 2-4 weeks to monitor growth. Tocolytics are generally ineffective for long-term prevention of preterm birth. Delivery timing is strictly guided by chorionicity: 37-38 weeks for dichorionic, 34-37 weeks for monochorionic, and 32-34 weeks for monoamniotic.
Prognosis
Primary complications include preterm delivery, preeclampsia, and gestational diabetes. Monochorionic twins require specialized monitoring for twin-twin transfusion syndrome and selective intrauterine growth restriction.
Differential Diagnosis
Polyhydramnios: excessive amniotic fluid volume
Uterine fibroids: firm, irregular mass on exam
Molar pregnancy: snowstorm pattern on ultrasound
Macrosomia: single fetus >4000g
Incorrect dating: most common cause of size-dates discrepancy