Multiple pregnancies carry higher risk. Specific expertise is a requisite for following such pregnancies.

In order to ensure that the most accurate calculation technique is used, the doctor will first have to establish the kind of multiple pregnancy he/she is dealing with.

Multiple pregnancies constitute almost 1% of all pregnancies. Two third of these are dichorionic (non-identical twins) while one third are monochorionic (identical).

  • Non-identical twins have separate placenta and separate amniotic sacs (dichorionic, diamniotic)
  • Identical twins have the same placenta (monochorionic), same amniotic fluid (monoamniotic) or some common organs (conjoined).
  • Subsequently, all monochorionic twins are identical and 6 out of 7 dichorionic twins are non-identical


Diagnosis of chorionicity
the best way to establish the chorionicity of a twin pregnancy by ultrasound at 11-13 weeks is by examining the junction between the membrane that separates the two fetuses and the placenta. In cases of dichorionic twins, you will be able to observe a triangular projection (lambda sign) of placental tissue inside the base of the membranes.

As the pregnancy progresses it becomes more difficult to identify the sign "lambda".
The outcome of the pregnancy is determined by whether the twins share the same placenta rather than whether they are identical or not.


Twins', Chromosomal abnormalities and Risk Calculation

  • Identical twins originate from the same group of cells that will break and create two different fetuses. As a result, a chromosomal abnormality will either affect either twins or neither of the two. The risk of chromosomal defect is linked to maternal age and is the same as that of a woman of the same age carrying a singleton.
  • In non-identical twins the risk of chromosomal abnormalities for each fetus is the same as in singleton pregnancies while the probability that at least one of the two fetuses is affected is double compared to a single pregnancy. This can be compared to throwing a dice: when throwing a dice the probability of getting the number 6 is 1 in six. However if you throw the dice twice, the probability of getting 6 at least once is equal to 1 in 6 plus 1 in 6, which is equal to 2 in 6 or 1 in 3.
  • An effective screening for chromosomal abnormalities in twin pregnancies is a combinaltion of maternal age, NT thickness, fetal heart beat and the level of free ß-hCG and PAPP-A in the maternal blood.
  • In diamniotic twin pregnancies, given that the measurements of the NT thickness may be different  for each twin, it is necessary to obtain a separate risk for trisomy 21 for each fetus. In these cases it is possible that one of the fetuses is healthy while the other is affected by a chromosomal abnormality.
  • In monoamniotic twin pregnancies, although the measurement of the thinkness of the fetuses’ can be different, the risk for chromosomal abnormalities is the same for both fetuses. The risk for trisomy 21 is calculated by taking into account the maternal age, NT thikness and maternal biochemistry. The average of the risk obtained for each fetus, will be considered as risk for the whole pregnancy.


Amniocentesis –Villocentesis in twins

  • Amniocentesis on twin pregnancies will give reliable results on the karyotype of both fetuses, with a risk of miscarriage of about 2%.
  • CVS in twin pregnancies also has 2% risk of miscarriage. However in almost 1% of cases, due to a double levy on the same placenta or cross-contamination of samples, a diagnostic error maybe encountered. Therefore amniocentesis would be the most preferred option.
  • However, in twin pregnancies where one of the fetuses are affected by a chromosomal abnormality while the other is normal, there is the possibility of having selective termination of the affected fetus or continue with the pregnancy with the hope that both fetuses will be born alive.
  • With selective termination (just one of the twins) after 16 weeks, the risk of miscarriage becomes three times higher compared to a selective termination before 14 weeks. Subsequently, if the parents decide to do a prenatal diagnostic test, it would be preferable to do a villocentesis. The result in this case will be available within 13 weeks of pregnancy while with amniocentesis results would only be available after 16-17 weeks of pregnancy.

Chorionicity and complications (BC= bicorial;MC= monocorial)


Miscarriage: In singleton pregnancies, where ultrasound shows a fetal viability of 11-13 +6 weeks, the probability of miscarriage or intrauterine fetal death before 24 weeks is about 1%. The risk of miscarriage in dichorionic twins is about 2% and in monochorionic twins of about 10%, the latter is mostly due to the twin-to-twin transfusion syndrome (TTTS).


Perinatal mortality: It is about 0.5% in singleton pregnancies, 2% in dichorionic twins and 4% in monochorionic twins. High mortality rate in multiple pregnancies is mostly due to complications that arise from premature delivery. In monochorionic twins, the risk is even higher due to twin-to-twin transfusion syndrome.


Growth restriction: In singleton pregnancies birth weight below the 5th percentile is generally 5%, in dichorionic twins is about 20% and in monochorionic twins of about 30%.


Pre-term deliver: Generally all babies born before the 24th week of pregnancy will die, while almost all those born after 32 weeks will survive. Babies born between 24 and 32 weeks are at high risk of neonatal death and disability. The risk of spontaneous delivery between 24 and 32 weeks is about 1% in single pregnancies, 5% in dichorionic twins and 10% in monochorionic twins.


Structural abnormalities: the likelihood of structural defects is about 1% in single pregnancies, 1% for each fetus in dichorionic twins and 4% for each fetus in monochorionic twins


Twin-to-twin transfusion syndrome

In all monochorionic pregnancies, there are vascular connections between the two fetuses in the placenta.

Twin-to-twin transfusion syndrome (TTTS) is caused by the blood flow through the placental vascular anastomoses from a fetus (donor) to another (the receiver).

In about 10% of monochorionic twin pregnancies severe TTTS, which is characterized by polyhydramnios (excess amniotic fluid) into the amniotic sac of the receiving fetus, can be observed within 16-24 weeks.

Another possible complication, which has been observed in approximately 10% of monochorionic twin pregnancies is severe growth restriction of one of the two fetuses.

Both TTTS and severe growth restrictions can be successfully treated with endoscopic laser surgery. This intervention breaks the connection between the blood vessels in the placenta. 

Both TTTS and severe growth restriction can be predicted by the difference in the measurement of NT between the two fetuses at 11-13 weeks. The risk for developing these complications is 30% higher in pregnancies where the difference in NT measurements is equal or more than 20%. The risk is reduced to 10% where the difference in NT is less than 20%






Last Edit: 05/08/2013 10:25pm