Discordant Fetal Growth

Definition

EFW discordance ≥ 20%, calculated as:

(EFW larger twin − EFW smaller twin) ÷ EFW larger twin × 100

• Mild discordance: 15–20% (considered a gray zone)
• Significant/severe discordance: ≥ 25–30% (strongly associated with adverse outcomes)

Etiology

Pathophysiology

• In dichorionic twins, the smaller (discordant) fetus most commonly suffers from selective fetal growth restriction (sFGR) due to a poorly implanted or smaller placental share.
• In monochorionic twins, vascular anastomoses create an additional layer of complexity — unequal placental sharing combines with inter-twin transfusion physiology. TTTS and TAPS must always be excluded.
• Chronic uteroplacental insufficiency leads to fetal hypoxia → redistribution of cardiac output (brain-sparing), progressive deterioration of Doppler indices, and eventually biophysical compromise.

Classification of Selective FGR in Monochorionic Twins (Gratacos Classification)

Diagnosis

Ultrasound Biometry

• Serial growth scans every 2–4 weeks in at-risk pregnancies
• Key measurements: BPD, HC, AC, FL → EFW by Hadlock formula
• AC discordance ≥ 20 mm is an early warning sign

Doppler Studies

1. Umbilical artery (UA): increased S/D ratio → absent end-diastolic flow (AEDF) → reversed end-diastolic flow (REDF)
2. Middle cerebral artery (MCA): low pulsatility index = brain-sparing; MCA PSV elevated = fetal anemia (TAPS)
3. Ductus venosus (DV): absent or reversed A-wave = imminent fetal compromise
4. Umbilical vein: pulsations = severe cardiac decompensation

Umbilical artery Doppler (Panel A) showing elevated RI in the smaller twin (Fetus A), with side-by-side abdominal circumference comparison (Panel B) demonstrating the discordant growth between twins.

Additional Workup

• Amniocentesis / CVS: if structural anomaly or chromosomal cause suspected
• Detailed anatomy scan: for congenital anomalies in the smaller twin
• Placental mapping: to assess cord insertion sites (MC twins especially)

Management

General Principles

• Manage based on chorionicity, gestational age, Doppler stage, and severity of discordance
• Multidisciplinary care at a fetal medicine unit

Monitoring Frequency

Timing of Delivery

Corticosteroids

• Administer betamethasone (two doses 24 hours apart) if delivery anticipated before 34–35 weeks for fetal lung maturity.

Interventional Options (Monochorionic Twins)

• Laser photocoagulation of placental anastomoses: primarily for TTTS, may also be considered in severe sFGR Type II/III in select cases
• Selective feticide (radiofrequency ablation / bipolar cord coagulation): considered when one twin has a lethal anomaly or when continuing the pregnancy threatens the co-twin's survival

Complications

Smaller Twin

• Intrauterine fetal demise (IUFD)
• Preterm birth and its sequelae
• Neonatal hypoglycemia, polycythemia
• Long-term neurodevelopmental impairment

Larger Twin (in MC pregnancies)

• Co-twin demise risk: acute hemorrhage into dead twin's placental territory → sudden neurological injury (periventricular leukomalacia, multicystic encephalomalacia) or co-twin death
• Risk is highest in Type III sFGR due to unpredictable acute arterio-arterial shunting

Both Twins

• Preterm labor
• TTTS (in MC twins)
• Iatrogenic prematurity

Prognosis

• DC twins: prognosis primarily determined by degree of discordance and Doppler status; mortality and morbidity rise sharply with EFW discordance ≥ 25% and abnormal Doppler
• MC twins: worse outcomes than DC due to shared circulation; Type II/III sFGR carry significant risks of IUFD and neurological damage even with intensive surveillance
• With appropriate fetal medicine center management, survival rates are reasonable but neurodevelopmental follow-up is mandatory for the smaller twin

Multiple Pregnancy

Definition & Incidence

• Twins: ~1 in 80 spontaneous pregnancies; much higher with ART
• Triplets: ~1 in 6,400 spontaneous; predominantly iatrogenic today
• Higher-order multiples: increasingly rare spontaneously

Zygosity and Chorionicity

Zygosity

Monozygotic Twin Types by Timing of Division

Determining Chorionicity

• Best assessed at 11–14 weeks by ultrasound
• Lambda (λ) sign = dichorionic (triangular wedge of placental tissue at membrane base)
• T-sign = monochorionic (thin membrane meets placenta at right angle, no wedge)
• Membrane thickness: DC > 2 mm; MC ≤ 2 mm

First-trimester ultrasound demonstrating the lambda sign — triangular placental tissue projecting into the inter-twin membrane — consistent with a dichorionic-diamniotic (DCDA) twin pregnancy.

Risk Stratification by Chorionicity

Maternal Complications

Obstetric

• Preterm labour and delivery (most common — ~50% of twins deliver before 37 weeks)
• Pre-eclampsia (2–3× increased risk)
• Gestational diabetes
• Placenta praevia and abruption
• Polyhydramnios
• Anaemia (increased iron and folate demands)
• Postpartum haemorrhage (uterine overdistension)

Non-Obstetric

• Hyperemesis gravidarum (elevated hCG)
• Urinary frequency, backache, varicosities (mechanical)
• Thromboembolic disease (increased hypercoagulable state)

Fetal/Neonatal Complications

Antenatal Care

First Trimester

• Early booking and dating scan
• Chorionicity determination at 11–14 weeks (critical — cannot be reliably assessed later)
• Combined first-trimester screening (NT + serum markers) — each fetus assessed separately
• NIPS/cfDNA: valid in twins; DZ twins usually discordant for trisomy; counsel about selective reduction if applicable

Supplements

• Folic acid 5 mg/day (from preconception to 12 weeks)
• Iron supplementation
• Low-dose aspirin (75–150 mg/day from 12 weeks) if pre-eclampsia risk high

Monitoring Schedule (NICE/RCOG-based)

Cervical Length

• Measurement at 18–22 weeks may predict preterm birth
• Cervical length < 25 mm → consider progesterone or cerclage in selected cases

Twin-to-Twin Transfusion Syndrome (TTTS)

Quintero Staging

Management

• Stages II–IV: Fetoscopic laser photocoagulation of anastomoses — treatment of choice at specialist centres
• Stage I: controversial; many centres monitor closely; some treat early
• Amnioreduction: palliative; may be used when laser is unavailable

TRAP Sequence (Twin Reversed Arterial Perfusion)

• Occurs in ~1% of MC pregnancies
• One twin is acardiac/acephalic ("acardiac twin"), perfused retrogradely by the pump twin
• Risk of pump twin heart failure and hydrops
• Treatment: radiofrequency ablation (RFA) or fetoscopic laser of the acardiac twin's cord

Delivery

Mode of Delivery

• Largely determined by the presentation of Twin 1
• Twin 1 cephalic: vaginal delivery is appropriate in experienced units (internal podalic version or breech extraction for Twin 2 is acceptable)
• Twin 1 non-cephalic: Caesarean section recommended

Timing of Delivery (NICE/RCOG Guidance)

Intrapartum

• IV access and crossmatch
• Continuous CTG monitoring of both fetuses
• Anaesthetic team on standby
• Oxytocin augmentation may be needed after delivery of Twin 1
• Active management of third stage (high PPH risk)

Higher-Order Multiples (Triplets and Beyond)

• Almost exclusively from ART
• Risks amplified proportionally (extreme prematurity, maternal morbidity)
• Multifetal pregnancy reduction (MFPR) to twins or singleton may be offered; reduces preterm birth and improves outcomes, at the cost of small miscarriage risk
• Delivery by Caesarean section is standard

Key Summary