San Gabriel Valley Perinatal                        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.
Dichorionic diamniotic (Di Di) 
Ultrasound findings  Two chorions (placentas) , two amniotic sacs, two yolk sacs . Lambda or "twin peak" sign (a triangular shape where each set of membranes meets) The lambda sign becomes difficult to identify after 20 weeks
Complications : Fetal growth restriction , preeclampsia, premature  rupture of their membranes, preterm labor, gestational diabetes, abruption, pyelonephritis,PUPPs ,thromboembolism, acute fatty liver, cesarean delivery, and  postpartum hemorrhage.
Evaluation of growth Every 4  weeks
Antenatal testing : Starting 32  weeks or sooner in all situations in which surveillance would ordinarily be performed in a singleton pregnancy
Delivery 38 weeks

Monochorionic diamniotic (Mo Di)
Ultrasound findings One chorion (placenta) , two amniotic sacs, two yolk sacs .T-shape where the membranes meet
Complications
In addition to the complications seen with DiDi twins MoDi twins are at increase risk for neural tube and heart defects, symmelia, extrophy of the cloaca,twin reversed arterial perfusion sequence (TRAP sequence),twin-to-twin transfusion syndrome  (TTTS) , and twin anemia-polycythemia sequence (TAPS)
Evaluation of growth : Every 3 to 4 weeks,
Evaluation for twin-to twin transfusion:  Every 2 weeks.

Antenatal testing: Starting at 32  weeks or sooner in all situations in which surveillance would ordinarily be performed in a singleton pregnancy
Delivery  34 to 37  weeks

Monochorionic monoamniotic (Mo Mo)
Ultrasound findings One  chorion (placenta) , one amniotic sac  one yolk sac.  No membrane. No T sign or lambda sign
Complications In addition to the complications seen with MoDi twins MoMo twins are at increased risk for  cord entanglement with an increased risk for stillbirth
Evaluation of growth every 3 to 4 weeks
Antenatal testing Hospital admission at 24 to 26 weeks with antenatal testing 2 to 3 times daily or continuous fetal monitoring
Delivery  32 to 34 w
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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
REFERENCES: http://www.obfocus.com/high-risk/twins/Twins.%20Your%20First%20Ultrasound%20Exam.htm
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