You've Got Twins! | |||||||
A twin pregnancy most commonly occurs when two
separate eggs are fertilized by separate sperm to form two babies that
develop separately. These are commonly called fraternal twins Each twin is surrounded by an outer membrane called the
chorion that forms the placenta , and an inner membrane called the amnion that
forms the amniotic sac (bag of waters) . Because there are two placentas
(dichorionic) and two amniotic sacs (diamniotic) the twins are called dichorionic diamniotic twins .Twins may also occur if a single fertilized
egg divides to form two separate babies. These twins are referred to
as "identical" twins. Depending on how soon after conception the
fertilized egg divides each twin may develop its own placenta and amniotic sac (dichorionic diamniotic)
, the twins may share one placenta (monochorionic) and develop in two separate
amniotic sacs (diamniotic), or the twins may share one placenta (monochorionic)
and share one amniotic sac (monoamniotic). Women carrying twin pregnancies are more likely to have preterm labor, small babies, a baby with a birth defect, preeclampsia, premature rupture of their membranes, gestational diabetes, and cesarean delivery. In addition identical twins are at increased risk for birth defects involving the brain and heart. Identical twins that share a placenta (monochorionic) may develop twin-to-twin transfusion syndrome (TTTS) or twin anemia-polycythemia sequence (TAPS). In addition, identical twins that also share the same amniotic sac are at risk for becoming entangled in each other's umbilical cord. Because of the different problems that may occur with twins that share a placenta or an amniotic sac the recommendations for monitoring and delivery of these different types of twin pregnancies are also different. It is important to identify the number of placentas and amniotic sacs as accurately as possible. The accuracy in identifying a twin pregnancy as monochorionic or dichorionic is highest when the ultrasound is performed before 14 weeks.
During an ultrasound examination about 70 to 80% of all twins will be expected to have two separate
placentas. About 20 to 30% of twins would be expected to share one
placenta and have separate amniotic sacs. A very small
number of twins (~1% ) would be expected to share one placenta and be in the
same amniotic sac. 5.4% of twins resulting from assisted reproductive
technology may be expected to be monochorionic. 3% of monochorionic placentas may have two lobes and appear to be
dichorionic |