o_1977r8vv9vk1ts2ms0kd8pksa.pdf

314 E.P. Gaziano, U. F. Harkness
May be abnormal in presence
of growth restriction, anomaly,
chromosome abnormality
Fig. 20.1. Type of placentation and relative risks for twin pregnancies. A amnion, C chorion, TTS twin-transfusion syndrome,
FGR fetal growth restriction
Fig. 20.2. Dividing membrane in dichorionic diamniotic
twin pregnancy. Note the anterior and posterior placentas
and relative thickness of the dividing membrane
diamniotic monochorionic gestation will result [8].
Monoamniotic monochorionic pregnancies occur
when the division occurs between days 8 and 13 after
fertilization [8]. Any pregnancy that splits later than
this develops into conjoined twins [8]. The type of
placentation is important since this factor defines the
relative risks for vascular anastomosis and the complications
that result in the greatest fetal morbidity.
Growth restriction may have a different mechanism
in fetuses from dizygotic pregnancies than in those
from a monozygotic pregnancy [9].
Figure 20.1 shows the relation of amnion to chorion
and the dividing membrane in diamniotic dichorionic
(DADC), diamniotic monochorionic (DAMC),
and monoamniotic monochorionic (MAMC) placentation;
the latter is rare, accounting for 1% of all twin
pregnancies [7]. The number of amnion/chorion
layers present in the dividing membrane is also
shown in Fig. 20.1. The ultrasonographic appearance
of the thicker diamniotic dichorionic dividing membrane
is apparent in Fig. 20.2. Monochorionic placentation
(20% of all twins) is associated with artery-toartery
(AA), artery-to-vein (AV), and vein-to-vein
(VV) superficial anastomosis, although deep capillary
villous anastomoses are also possible (Fig. 20.3) [5, 7,
10]. All gradations of anastomoses are possible;
hence, the variable clinical presentations and variable
Doppler velocimetry findings observed. Most cases of
twin transfusion occur with diamniotic monochorio-