Functional assessment and volume together
In case of twins, Doppler testing is prescribed more often than in singleton pregnancies: the indications are multiple pregnancy itself, differences in the size of the fetuses, monochorionic type of placentation, oligohydramnios or polyhydramnios in one of the sacs, arterial hypertension and preeclampsia in the mother. Measurements are performed separately for each fetus - in the umbilical artery, middle cerebral artery and, if necessary, in the ductus venosus, and blood flow in the uterine arteries is also assessed.
The key point of separate measurement is that with multiple births, the fetuses can be in completely different states. One grows normally and has normal indices, the second lags behind and shows signs of centralization of the blood circulation. An average estimate here is useless and can be misleading.
The volumetric part of the study is performed after completion of the functional program, when all measurements are entered into the protocol. This is a convenient format for families: in one visit, a full medical monitoring of the children’s condition and shooting of three-dimensional images with video recording are carried out.
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.