Monozygotic twin pregnancies, the result of a single fertilised egg splitting into two embryos, characteristically demonstrates two foetuses in the same gestational sac in the first trimester on ultrasound.
Description
Monozygotic twin pregnancy, often referred to as an identical twin pregnancy, arises from a single fertilised egg (zygote) which subsequently divides into two separate embryos. These twins share the same genetic material and are always of the same sex. Depending on the timing of the split, they may share the same placenta (monochorionic) or have separate placentas (dichorionic).
Pathogenesis
Monozygotic twinning occurs when a single zygote undergoes division. The exact cause for this division is unclear, but it is believed to be a random event. The timing of this division affects the chorionicity and amnionicity of the pregnancy:
- Division within 72 hours results in dichorionic diamniotic twins.
- Division between 4 to 8 days results in monochorionic diamniotic twins.
- Division after 8 days results in monochorionic monoamniotic twins, and
- Division after 13 days leads to conjoined twins.
Subtypes
Monozygotic twin pregnancies are classified into three main subtypes, based on the chorionicity and amnionicity:
- Dichorionic diamniotic (DCDA)
- Monochorionic diamniotic (MCDA)
- Monochorionic monoamniotic (MCMA)
Epidemiology, Risk Factors & Associations
- Monozygotic twinning occurs in approximately 3-4 per 1000 pregnancies worldwide.
- There are no well-established risk factors for monozygotic twinning; however, the use of assisted reproductive technologies has been associated with an increased incidence.
- Associated complications include twin-to-twin transfusion syndrome, twin reversed arterial perfusion sequence, and twin anemia-polycythemia sequence.
Clinical Features
Clinical features are largely similar to other pregnancies but may have more pronounced symptoms due to increased hormonal production. These can include increased morning sickness and more rapid uterine growth.
Complications
Complications associated with monozygotic twin pregnancies include a higher risk of preterm delivery, intrauterine growth restriction, preeclampsia, and congenital abnormalities. There is also a risk of specific conditions like twin-to-twin transfusion syndrome (TTTS), twin anemia-polycythemia sequence (TAPS), and twin reversed arterial perfusion sequence (TRAPS) in monochorionic pregnancies.
Pathological Features
Histopathology
- Macroscopic: In early pregnancy, two fetuses can be seen within the same gestational sac.
- Microscopic: Not applicable.
Serology
- Elevated levels of human chorionic gonadotropin (hCG) and alpha-fetoprotein (AFP) may be seen in twin pregnancies.
Biochemistry
- No specific biochemical markers exist for monozygotic twin pregnancies.
Radiological Features
General Features
- Monozygotic twin pregnancies can be identified on ultrasound by the presence of two foetuses sharing the same gestational sac (in early pregnancy) or placenta.
Ultrasound
- B-mode: Demonstrates two feotuses in the same gestational sac in the first trimester.
- Doppler: Can identify shared placental circulation in monochorionic pregnancies, important for diagnosing complications such as TTTS.
Diagnosis
Diagnosis is generally made through ultrasound examination. A detailed first-trimester scan can determine chorionicity and amnionicity, which are crucial for predicting potential complications and guiding management.
Differential Diagnosis
- Dizygotic twin pregnancy: Distinguished by the presence of two separate gestational sacs and placentas on ultrasound.
Management
Management of monozygotic twin pregnancy often involves more frequent antenatal visits and ultrasound examinations to closely monitor for complications, particularly in monochorionic pregnancies. Referral to a specialist in maternal-fetal medicine is typically recommended.