Multiple pregnancies are categorised into mono- and di- or trichorionic. For this classification, it is important to vizualise the membranes before 12 gestational weeks using ultrasound. In dichorionic multiple pregnancy, each child has its own placenta and the dividing membrane appears very thick. Should the dividing membrane be rather thin, the multiplets are monochorionic and share one placenta. If there is no dividing membrane, the multiplets are called monochorionic monoamniotic. The latter share a placenta and carry further risks. Therefore, we recommend referring these pregnancies in qualified centres.

In general, all monochorionic multiples are monozygotic. In monochorionic multiplets, it is recommended to have an ultrasound scan done every 2 weeks. The examination always includes various Doppler measurements in fetal vessels.

Multiple pregnancies have an increased risk of premature birth. Regardless of the membrane’s constellation, the cervical length should  be measured from 16 gestational week onwards every approx. 4 weeks (see section preterm birth).

Dichorionic twin gestation

If each multiplet has its own placenta, the placental distribution may still differ. Should this be significant, the multiplet of the smaller parts grows more slowly; hence the weight discrepancy between the two children increases. In case of impending of oxygen deficiency as suspected by ultrasound, all multiples may have to be delivered prematurely.

Monochorionic twin gestation

Monochorionic multiplets share a common placenta with vascular connections. This has certain consequences:

1. In a minority of cases, a slow transfusion from one (donor) to another multiplet (acceptor) may lead to a chronic twin-to-twin-transfusion syndrome (TTTS). It is a disease that usually occurs in the second trimester of pregnancy and can be detected by ultrasound by a discrepancy of the amniotic fluid Therefore, it is also called twin oligohydramnious-polyhydramnious sequence (TOPS). Once suspected, these pregnancies should be followed up in a specialised center to decide whether there is a need for an intrauterine intervention, such as  a fetoscopic laser coagulation of the connecting vessels Studies have shown significantly improved outcome of multiplets, provided the procedure was performed by qualified specialists with  a sufficient number of cases per year.
2. A subacute transfusion syndrome is less common. The onset is later after 24 gestational weeks and may be fast. Thus, it is called twin anemia-polycythemia sequence (TAPS). The specific characteristics are anemia in one and polycythemia in the other multiplet. Most frequently, rapid delivery in a perinatal center with prepared neonatologists for eventual transfusion  is the only option.
3. In some cases, a monochorionic placenta can be unevenly distributed between the multiplets. The surveillance and management requires high expertise and should therefore be performed by specialised centres.

Information that is more detailed can be found on our information leaflet.