Dizygotic twins, the most common type of twins, occur when two separate eggs are fertilised by different sperm, characterised by the presence of two separate gestational sacs and placentas.
Description
Dizygotic twins, also known as fraternal twins or non-identical twins, are a type of multiple pregnancy where two separate eggs are fertilised by two different sperm. Each foetus develops independently with its own placenta and amniotic sac.
Pathogenesis
Dizygotic twins occur when two eggs are released during ovulation and fertilised by separate sperm. This is distinct from monozygotic twins, which develop from a single fertilised egg that later splits into two embryos. This always results in dichorionic-diamniotic pregnancies.
Epidemiology, Risk Factors & Associations
Dizygotic twinning is more common than monozygotic twinning, accounting for approximately 70% of twin pregnancies.
Risk factors include:
- Advanced maternal age
- Maternal family history of twinning
- Certain ethnic backgrounds (e.g., African and Nigerian descent)
- Assisted reproductive technologies (IVF, particularly use of ovulation induction agents)
Clinical Features
Dizygotic twins present with typical features of a multiple pregnancy:
- An enlarged uterus
- Increased weight gain
- Elevated human chorionic gonadotropin (hCG) levels
Complications
The risk of perinatal mortality is lower (10%) compared to monozygotic twin pregnancies, particularly the monochoronic-monoamniotic subtype (>50%).
Pathological Features
Histopathology
- Macroscopic: Macroscopic examination typically reveals two separate placentas and amniotic sacs.
- Microscopic: Microscopic examination may not be relevant in the context of dizygotic twins.
Radiological Features
General Features
- Dizygotic twins appear as two separate gestational sacs and placentas on ultrasound.
- Differentiating imaging appearances between the foetuses may be challenging, and individual foetal anatomy should be assessed independently. Visualisation of different sexes is a reliable sign of dizygosity.
- No characteristic imaging findings are specific to dizygotic twins.
US
- B-mode: Visualisation of two separate gestational sacs and placentas.
- Colour Doppler: Assessing blood flow in each foetus and placenta.
Diagnosis
The diagnosis of dizygotic twins is typically made during routine obstetric ultrasound examinations, where the presence of two separate gestational sacs and placentas is visualised.
Differential Diagnosis
There is no specific differential diagnosis for dizygotic twins as their appearance on imaging is characteristic.
Management
Pregnancies with dizygotic twins are usually managed by an obstetrician specialising in multiple pregnancies. Regular monitoring, including ultrasound examinations and maternal-foetal assessments, is necessary to ensure the well-being of both foetuses throughout the pregnancy.