Twin Pregnancy and Chorionicity — Timing of Splitting, Lambda and T Signs, TTTS

Chorionicity is the number of placentas (chorions) in a multiple pregnancy; amnionicity is the number of amniotic sacs. Zygosity is whether the twins came from one egg (monozygotic) or two (dizygotic). Much of the risk in a twin pregnancy depends on chorionicity: NICE guideline NG137 tells trainers to stress that the risks of twin and triplet pregnancy are determined by chorionicity and not zygosity.

About 67% of twin gestations are dizygotic and 33% monozygotic. Among monozygotic twins roughly two-thirds are monochorionic (one shared placenta). Twin births make up about 2% to 4% of births worldwide.

In the most accepted model of monozygotic twinning, the number of placentas and amniotic sacs depends on when the zygote splits:

The best time to determine chorionicity is the first trimester; ISUOG says it should be done before 13+6 weeks. StatPearls recommends an ultrasound at 10 to 13 weeks for chorionicity, crown-rump length and nuchal translucency, and NICE advises offering a first-trimester scan to estimate gestational age and determine chorionicity and amnionicity, ideally at the same scan.

• Lambda (λ) or twin-peak sign — the twins are separated by a thick layer of fused chorionic membranes with a thin amniotic layer on each side (the 'full lambda'). It is seen in dichorionic twins because the dividing membrane has four layers (two chorions and two amnions).
• T sign (also called the 'empty lambda' sign) — the twins are separated by only two thin amniotic layers. It is seen in monochorionic diamniotic twins. ISUOG notes the base of insertion may still look triangular in monochorionic twins, but it contains no chorion and must not be mistaken for the full lambda sign.
• ISUOG advises examining the entire intertwin septum at its insertion into the placenta, and using as many features as possible: membrane thickness, number of placental masses and the lambda or T sign. Keep an image showing chorionicity in the records.

If the woman presents after 14+0 weeks, NICE says to determine chorionicity at the earliest opportunity using all of: the number of placental masses, the presence of amniotic membrane(s) and membrane thickness, the lambda or T sign and discordant fetal sex. If chorionicity cannot be determined even after referral (for example, a late booking), manage the pregnancy as monochorionic until proven otherwise.

Early diagnosis of chorionicity decides the surveillance schedule. For uncomplicated twin pregnancies NICE NG137 recommends:

StatPearls gives the rationale: once an MCDA pregnancy is established, ultrasound surveillance for TTTS begins at 16 weeks and continues at least every 2 weeks until delivery, checking the amniotic fluid on both sides of the membrane and the presence of a urine-filled bladder in each twin, with umbilical artery and, in recent guidance, middle cerebral artery Doppler.

TTTS is a complication of monochorionic placentation. More than 95% of MCDA twins share an intertwin placental circulation. TTTS occurs in about 8–12% of MCDA and about 6% of MCMA pregnancies. Its cause is unbalanced arteriovenous anastomoses deep in the placental cotyledons, shunting blood from the donor to the recipient. Arterio-arterial anastomoses are protective, which is why TTTS is less frequent in MCMA twins, whose placentas typically have more of them.

• Donor twin: hypovolaemia → renal hypoperfusion → RAAS activation → oliguria → oligohydramnios (the amnion may look 'stuck' to the fetus).
• Recipient twin: hypervolaemia → cardiac stretch and raised ANP/BNP → polyuria → polyhydramnios; valve insufficiency, diastolic dysfunction and pulmonary stenosis or atresia can follow.
• Diagnosis: in an MCDA pregnancy, one sac with maximum vertical pocket (MVP) < 2 cm and the other with MVP > 8 cm. Growth discordance may occur but is not required.

Management depends on stage and gestation. Stage I without symptoms: expectant management with at least weekly surveillance (about 25% progress). Stages II to IV between 16 and 26 weeks: fetoscopic laser photocoagulation of the anastomoses, which gave better survival and neurological outcomes than serial amnioreduction in the Senat randomised trial. The Solomon technique coagulates a line along the whole placental equator after the anastomoses; it reduces recurrence and TAPS but carries a greater risk of abruption. Laser before 16 weeks increases the risk of premature rupture of membranes; after 26 weeks coagulation is harder because the vessels are larger.

Twin anaemia-polycythaemia sequence (TAPS) results from tiny (submillimetre) arteriovenous connections near the placental edge, allowing slow transfusion of about 5–15 mL per day: one twin becomes anaemic (donor) and the other polycythaemic (recipient), without the oligo-polyhydramnios of TTTS. MCA Doppler is the diagnostic clue. It occurs spontaneously in about 2–5% of MCDA twins and can follow laser therapy for TTTS.

NICE advises timing of planned birth for uncomplicated twin pregnancies as follows:

• Image-based: identify the lambda sign (dichorionic) or T sign (monochorionic) and state the placentation.
• Timing of splitting: 3, 8 and 13 days — which placentation results?
• 'Best time to determine chorionicity' → first trimester (10 to 13 weeks).
• Twin pregnancy with oligohydramnios in one sac and polyhydramnios in the other → TTTS; treatment of Stage II–IV → fetoscopic laser.
• Scan frequency in MCDA twins → every 2 weeks from 16 weeks.
• Most dangerous twin type → monochorionic monoamniotic.