dichorionic diamniotic twin pregnancy
A dichorionic diamniotic (DCDA) twin pregnancy is a type of twin pregnancy where each twin has its own chorionic and amniotic sacs. This type occurs most commonly with dizygotic twins, but may also occur with monozygotic twin pregnancies. This type of pregnancy may have characteristic findings on ultrasound.
DCDA pregnancies account for the majority (~76%) of all twin pregnancies. They account for all dizygotic pregnancies and ~20% of monozygotic pregnancies.
With a dizygotic pregnancy, two ova are independently fertilised by two sperm leading to two zygotes.
With a monozygotic twin pregnancy, DCDA pregnancy results from the separation of the zygotes at ~1-4 days post fertilisation (morula) stage.
Sonographic assessment of chorionicity is most accurate in the first trimester.
Features supporting a DCDA pregnancy:
- presence of two gestational sacs with a thick echogenic chorion surrounding each embryo
- a thick inter-twin membrane (often taken as > 2 mm)
- twin peak sign (lambda (λ) sign)
- two yolk sacs may be seen (this, however, does not differentiate a DCDA pregnancy from a monochorionic diamniotic (MCDA) pregnancy)
- when there is no placental fusion, two separate placental sites may be seen
- a finding of two different genders for each twin is a definitive feature for a dizygotic pregnancy which in turn will invariably mean a DCDA pregnancy
If the twins are of the same gender then it is extremely difficult if not impossible to determine if they are monozygotic or dizygotic on ultrasound.
While the complication rate is still much higher with twins than with a singleton pregnancy, a DCDA pregnancy carries the lowest rate of complications amongst twin pregnancies. Such recognized complications include: