Doppler for monochorionic twins
In case of monochorionic twins, Doppler measurement turns from an additional method into the main control tool. The fetuses are connected by vascular anastomoses within the common placenta, and an imbalance of blood flow between them manifests itself earlier and more dangerously than with dichorionic twins.
Indices are measured in the umbilical cord artery of each fetus - they are used to evaluate the resistance of the placental bed and identify selective growth retardation. Particular attention is paid to the nature of the diastolic component: its absence or reverse blood flow, especially of the intermittent type, indicates a pronounced imbalance of the anastomoses. Peak systolic velocity is measured in the middle cerebral artery: multidirectional changes in this indicator in fetuses are a sign of anemia-polycythemic sequence, in which one fetus becomes anemic and the second becomes polycythemic.
Blood flow in the ductus venosus reflects the state of fetal cardiac function and changes with volume overload in the recipient fetus in feto-fetal transfusion syndrome. The combination of these indicators, together with the difference in the volume of amniotic fluid, determines the stage of the syndrome and the need to refer the anastomoses for laser coagulation.
Chorionicity is the main issue in multiple pregnancy
The prognosis of a multiple pregnancy is determined not so much by the number of fetuses as by the number of placentas and membranes. Dichorionic twins, where each fetus has its own placenta, proceeds relatively well. Monochorionic, where there is one placenta for two, carries the risk of specific complications associated with vascular connections within the common placenta, and requires monitoring every two weeks.
Chorionicity is most reliably determined at 11–14 weeks by the shape of the septal attachment site: the λ-sign indicates dichorionic twins, the T-sign indicates monochorionic twins. After 16–18 weeks, these signs smooth out, and the accuracy of the determination drops sharply. That is why the first study in case of multiple pregnancy is especially important and cannot be postponed.
Specific complications
With monochorionic multiple pregnancy, feto-fetal transfusion syndrome is possible - uneven redistribution of blood between fetuses through anastomoses of the common placenta. One fetus receives excess volume and polyhydramnios, the second receives less and suffers from oligohydramnios and growth retardation. The condition develops quickly and requires timely intervention, so the main way to detect it is regular ultrasounds with measurement of water pockets in each fetus.
The second common problem is selective growth retardation, when one fetus is significantly behind the other. A difference in the calculated mass of more than 20–25 percent is considered significant. Anemia-polycythaemic sequence is also possible, which is detected by the difference in peak systolic velocity in the middle cerebral arteries of the fetuses.
Why does the study take longer?
In case of multiple pregnancy, not one study is performed, but several: a full protocol for each fetus plus a general assessment of the uterus, placenta, septum and cervix. The volume of measurements increases as a multiple of the number of fetuses, hence the longer duration of treatment and higher cost compared to a singleton pregnancy.
An additional complexity is created by the relative position of the fruits: they cover each other, and the doctor has to spend a long time looking for the right cut. Sometimes part of the protocol is postponed for a follow-up visit a few days later - this is common practice and not a sign of trouble.