Features of the volumetric mode for twins
A three-dimensional reconstruction is built along the boundary between the surface of the fetus and the amniotic fluid. With twins, there is less free space, the fetuses are pressed against each other and against the walls of the uterus, and the interfetal septum further limits the viewing angle. Therefore, it is not always possible to get equally successful shots of both children in one shot.
Practice shows that it is better to come at 24–28 weeks - a little earlier than with a singleton pregnancy. By 30 weeks, twins are already so crowded that their faces are most often pressed together. The location of the placenta also matters: if the placenta is on the anterior wall, it blocks access and the image quality decreases.
A separate pleasant feature of multiple births is the opportunity to shoot footage of children touching each other with their hands or lying face to face. Such angles are not always obtained and depend on luck, so it is worth setting aside some time for research and being prepared to wait a little until the children find a comfortable position.
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.