What is the medical illustration and management of dizygotic (fraternal) twins?

Dizygotic (Fraternal) Twins: Medical Illustration and Management

Dizygotic twins result from fertilization of two separate oocytes by two separate sperm, creating nonidentical or "fraternal" twins that nearly always have dichorionic-diamniotic placentation with two separate placental masses and two amniotic sacs. 1

Biological Basis and Placentation

Dizygotic twins are fundamentally different from monozygotic twins in their conception and placental architecture:

• Two separate fertilization events occur when two oocytes (from one or two Graff follicles) are fertilized by two different sperm 2
• Dichorionic-diamniotic placentation is the hallmark, with two distinct gestational sacs, two separate placental masses, and two amniotic sacs 1
• No shared placental circulation exists between the twins, eliminating risks of twin-twin transfusion syndrome and twin anemia-polycythemia sequence that affect monochorionic twins 1

Rare Exception to Note:

• Monochorionic-dizygotic twins have been reported in rare cases (particularly with assisted reproductive technology), though the mechanism remains unclear 1, 3, 4
• These unusual cases can result in blood chimerism (demonstrable in 90.3% of cases) and diagnostic difficulties 3
• Assisted reproductive technology is responsible for 82.1% of reported monochorionic-dizygotic twin cases 3

Ultrasound Identification

First-trimester ultrasound achieves nearly 100% accuracy in determining chorionicity by counting gestational sacs:

• At 6 weeks gestation: Two distinct gestational sacs are visible sonographically 2
• At 7-8 weeks gestation: Two embryos with fetal heart rates can be visualized 2
• At 8 weeks: Two distinct gestational sacs confirm dichorionic placentation (Figure 2 in guidelines shows this classic appearance) 1

Key Sonographic Features (10-14 weeks):

• "Lambda" or "twin peak" sign: Triangular projection of placental tissue (chorionic villi) into the base of the intertwin membrane, with sensitivity 97-99% and specificity 95-100% 1
• Two separate placental masses visible 1, 5
• Thicker dividing membrane compared to monochorionic twins 1, 5
• Discordant fetal sex (male and female) definitively confirms dizygotic twins and dichorionic placentation 1

Important caveat: The lambda sign becomes less reliable beyond the first trimester due to regression of chorion frondosum, so early determination is crucial 1

Clinical Management and Surveillance

Dichorionic twins (including dizygotic twins) have significantly better outcomes than monochorionic twins:

• Perinatal mortality rate is more than 2-fold lower in dichorionic twins compared to monochorionic twins (1.6% vs 7.6% fetal demise rate) 1
• Neonatal morbidity is reduced, with lower rates of necrotizing enterocolitis 1
• Lowest incidence of preterm birth, perinatal mortality, and birth weight discordance among all twin types 6

Recommended Surveillance Protocol:

First trimester:

• Dating and chorionicity determination scan 7
• Nuchal translucency scan at 11-14 weeks 7

Second trimester:

• Detailed anatomic survey at 18-22 weeks 7
• Note: Congenital anomalies occur in 1 in 25 dichorionic twins (lower than monochorionic twins at 1 in 15) 7

Third trimester:

• Regular growth scans 7
• Monitor for placenta previa (more common in dichorionic twins) 1, 7

Size Discordance Monitoring:

• Crown-rump length (CRL) discordance <20% in first trimester carries 3% risk of subsequent demise 1
• CRL discordance >60% carries 100% risk of demise 1
• Estimated fetal weight discordance ≥25% warrants closer monitoring 8
• Use the larger twin for dating purposes to minimize missing growth restriction 1

Risk Factors and Complications

Dizygotic twinning is influenced by:

• Maternal race, age, and parity 2
• Clomiphene citrate and gonadotropin use for ovulation stimulation 2
• Discontinued contraceptive pill use 2
• Seasonal factors (sunlight exposure) 2

Pregnancy complications include:

• 5-fold increase in fetal death and 7-fold increase in neonatal death compared to singletons (primarily from prematurity) 7
• More frequent spontaneous abortions 2
• Hypertension in pregnancy 2
• Hemorrhage, anemia, early rupture of membranes, hydramnios, premature delivery 2

Vanishing Twin Phenomenon:

• One gestational sac may be resorbed during pregnancy while the other fetus develops normally 2
• May occur without clinical symptoms or with light hemorrhage 2
• Requires follow-up ultrasound to confirm continued viability of remaining twin 2

Critical Management Distinctions

Unlike monochorionic twins, dizygotic (dichorionic) twins do NOT require:

• Middle cerebral artery Doppler surveillance for twin anemia-polycythemia sequence 1
• Weekly ultrasound monitoring for twin-twin transfusion syndrome 1
• Referral to specialized fetal intervention centers (unless other complications arise) 1
• Fetoscopic laser surgery 1

The absence of shared placental circulation eliminates the unique complications of monochorionicity, making dizygotic twin management more straightforward than monozygotic monochorionic twin management. 1