Multiple gestations consist of 1% of all gestations with twins corresponding to 95% of multiple gestations. Multiple gestations entail higher risk for developing antenatal and perinatal complications. It is known that 20% of premature births are associated with twin gestations. Therefore, a close antenatal surveillance is necessary to reduce the risk for the mother and the fetuses.
Risk factors for multiple gestations
- Family history, heredity derives primarily from the mother and much less form the father
- Mother’s age greater than 35 years
- Multi parity (more than 4 births)
- Ovulation induction (citric clomiphene)
- In Vitro Fertilization (IVF)
Chorionicity and amnionicity
Chorionicity (number of placentas) and amnionicity (number of amniotic sacs) have to be determined as soon as possible during pregnancy (usually until the 12th week of gestation) because they indicate the antenatal management.
Twin gestations are distinguished with respect to:
- The number of placentas
Dichorionic (every embryo has its own placenta)
Monochorionic (a single placenta that is common for both embryos)
- The number of amniotic sacs
Diamniotic (each embryo has its own sac)
Monoamniotic (both embryos develop in the same sac)
- The number of placentas
Chorionicity is determined with ultrasound examination at the 12th week of gestation. Monochorionic twins are separated by a thin membrane forming the letter “T” at the insertion of the placenta. Dichorionic twins have two different placentas and amniotic sacs and they are separated by a thicker membrane forming the letter “Λ” (lamda sign). Twins of different gender are always dichorionic.
Antenatal care of dichorionic twins
Dichorionic twins relate to a higher risk of premature birth (about 10% possibility of birth before 32 weeks) and increased risk (about 10%) for intrauterine growth restriction. Close ultrasound monitoring is advised beginning form 12 weeks at 22, 28, 32 and 36 weeks.
Antenatal care of monochorionic twins
In monochorionic twins there is 15% possibility for developing a serious Twin-To-Twin Transfusion Syndrome (TTTS) or Selective Intrauterine Growth Restriction (sIUGR) and an increased probability of premature birth. A thorough ultrasound examination is suggested at 12 weeks for detecting early signings of TTTS or sIUGR syndromes and then regular ultrasound examinations after the 16th week. Delivery is planned at 35-36 weeks if there are no complications. If signs of serious TTTS or sIUGR do exist, a more specialized treatment could be applied, in consultation with the parents.