20.3 Multiple Gestations, Assisted Reproduction & Maternal Disease

Determining Chorionicity and Amnionicity

The single most important task in twin sonography is establishing chorionicity (number of placentas) and amnionicity (number of amniotic sacs), because chorionicity - not zygosity - drives risk. The first trimester is the best time to make the determination: count the gestational sacs, each surrounded by its own thick echogenic chorionic ring, to establish chorionicity, and count yolk sacs and amniotic membranes for amnionicity. The number of yolk sacs usually parallels the number of amnions, although rare exceptions exist, so membrane counting later in gestation remains the arbiter.

After about 10-12 weeks, the classic membrane signs apply:

• Twin peak (lambda) sign: a triangular wedge of chorionic tissue projecting between the layers of the dividing membrane at its placental insertion - indicates dichorionic twins.
• T-sign: a thin membrane meeting the placental surface at a 90-degree angle with no tissue wedge - indicates monochorionic twins.
• A thick dividing membrane (greater than about 2 mm) supports dichorionicity; a hair-thin membrane supports monochorionicity. Membrane thickness alone is less reliable than the peak and T signs.

Twin types and their risks

Twin-to-twin transfusion syndrome (TTTS)

TTTS complicates roughly 10-15% of monochorionic-diamniotic pregnancies when unbalanced placental arteriovenous anastomoses shunt blood from one twin to the other. The donor twin becomes hypovolemic and growth-restricted with oligohydramnios - a deepest vertical pocket under 2 cm - and may become a stuck twin plastered against the uterine wall with a small or invisible bladder. The recipient twin becomes hypervolemic with polyhydramnios (a deepest pocket over 8 cm, and over 10 cm late in gestation), cardiomegaly, and eventually hydrops. Quintero staging: Stage I, the oligo-poly fluid sequence only; Stage II, the donor bladder is not visible; Stage III, critically abnormal Dopplers (absent or reversed umbilical artery end-diastolic flow, reversed ductus venosus a-wave, or a pulsatile umbilical vein); Stage IV, hydrops; Stage V, demise of one or both twins. Fetoscopic laser ablation of the anastomoses is the definitive therapy for qualifying stages at appropriate gestational ages.

Selective fetal growth restriction (sFGR) is diagnosed when one twin falls below the 10th percentile (severe: below the 3rd) or the estimated weight discordance exceeds about 20-25%. A vanishing twin - resorption of one of two first-trimester embryos - occurs in up to 20-30% of early twin conceptions and is usually benign for a dichorionic survivor, whereas in utero demise of a monochorionic co-twin threatens the survivor with hypotensive and embolic neurologic injury through the shared circulation. Conjoined twins result from incomplete splitting at day 13 or beyond; thoracopagus (joined chests with a shared heart) is the most common form and carries the poorest prognosis.

Assisted reproduction and implantation

Assisted reproductive technology (ART) - ovulation induction, intrauterine insemination, and in vitro fertilization (IVF) - has reshaped multiple-gestation epidemiology. A large share of twins and nearly all higher-order multiples are now iatrogenic, and IVF slightly increases monozygotic splitting as well. ART also raises the stakes of early scanning:

• Ectopic pregnancy occurs in roughly 2-5% of ART conceptions, and heterotopic pregnancy - simultaneous intrauterine and ectopic gestations - occurs in about 1 in 100 IVF pregnancies versus roughly 1 in 30,000 spontaneous ones. Always survey the adnexa even after confirming an intrauterine sac in an ART patient.
• Implantation abnormalities are more frequent: low implantation and placenta previa, as well as the accreta spectrum, particularly with prior uterine surgery.

Maternal diabetes

Glucose crosses the placenta but insulin does not, so maternal hyperglycemia drives fetal hyperinsulinism - and insulin is the major fetal growth hormone. Pregestational diabetes exposes the embryo during organogenesis and is teratogenic: cardiac defects, neural tube defects, renal anomalies, and caudal regression syndrome (sacral agenesis), which is rare but highly specific to diabetic embryopathy. Later in gestation, both pregestational and gestational diabetes produce macrosomia - an estimated fetal weight above the 90th percentile or above 4000-4500 g - with disproportionate abdominal circumference and shoulder girth (raising shoulder dystocia risk), polyhydramnios, and delayed fetal lung maturity. Because gestational diabetes begins after organogenesis, it does not raise the anomaly rate - a distinction the exam tests directly.

Hypertensive disorders

Chronic hypertension and preeclampsia reflect failed spiral-artery remodeling and uteroplacental insufficiency. Sonographic consequences include (often asymmetric) fetal growth restriction, oligohydramnios, elevated umbilical artery resistance, and an increased risk of placental abruption. Persistent uterine artery notching after 24-26 weeks flags the highest-risk patients, as covered in the Doppler section.

Uterine anomalies

Mullerian duct anomalies complicate pregnancy and are best characterized with 3D ultrasound:

• Septate uterus: normal convex external fundal contour with an internal fibromuscular septum (failed resorption after normal fusion) - the anomaly with the highest miscarriage rate; treatable by hysteroscopic septum resection.
• Bicornuate uterus: incomplete fusion producing two horns, one cervix, and an external fundal cleft deeper than about 1 cm.
• Uterus didelphys: complete fusion failure with two uteri, two cervices, and often a longitudinal vaginal septum.
• Unicornuate uterus: a single horn, often with a rudimentary horn; pregnancy in a non-communicating rudimentary horn risks catastrophic rupture.

All of these anomalies raise the rates of miscarriage, malpresentation (especially breech), preterm birth, growth restriction, and retained placenta.

TAPS and TRAP: The Other Monochorionic Complications

Twin anemia-polycythemia sequence (TAPS) is a chronic, slow inter-twin transfusion across tiny arteriovenous anastomoses that produces a large haemoglobin discordance without the amniotic fluid discordance that defines TTTS. Because the fluid volumes look normal, TAPS is missed unless Doppler is applied deliberately: the diagnosis rests on middle cerebral artery peak systolic velocity, with the donor above 1.5 MoM (anaemic) and the recipient below 1.0 MoM (polycythaemic). It arises spontaneously in roughly 3 to 5 percent of monochorionic pregnancies and in up to 13 percent after laser therapy for TTTS.

Twin reversed arterial perfusion (TRAP) sequence, or acardiac twin, is the extreme of monochorionic sharing. A large artery-to-artery anastomosis lets the normal pump twin perfuse a non-viable co-twin in reverse — blood enters the acardiac twin through its umbilical artery rather than its vein. The acardiac twin has no functioning heart and often no upper body, appearing as an amorphous oedematous mass that nonetheless enlarges over time. The sonographic signature is reversed arterial flow directed toward the acardiac mass on Doppler. It is the pump twin who is at risk — of high-output cardiac failure and hydrops — and that risk climbs as the acardiac mass approaches the pump twin's weight.

Maternal Anatomy in the Second and Third Trimester

The blueprint lists maternal uterus, cervix, and ovaries, and the last two are easy to neglect once attention shifts to the fetus.

• Cervix. Assess length whenever a risk factor for preterm birth is present; transvaginal measurement is the reference standard, and a length under 25 mm before 24 weeks is short (see Section 19.3).
• Ovaries. The corpus luteum of pregnancy is the expected first-trimester finding and normally involutes by about 16 weeks. A persistent or new adnexal mass needs characterisation: theca lutein cysts — bilateral, large, multiseptated — accompany the high hCG of gestational trophoblastic disease, multiple gestation, and ovarian hyperstimulation, whereas a luteoma is a solid, hormonally active benign mass that regresses after delivery. Adnexal torsion is more common in pregnancy, particularly following ovulation induction.
• Uterus. Document fibroid location and size when present: lower-segment fibroids can obstruct delivery, and fibroids commonly undergo painful red (carneous) degeneration during pregnancy.