How to choose a day for research
With twins, the window in which a truly good three-dimensional image is obtained is noticeably narrower than with a singleton pregnancy, and closes earlier. A practical guideline is the second half of the second trimester and the very beginning of the third. By this time, the fetuses have already developed subcutaneous fatty tissue and their faces look rounded, but the uterus has not yet been stretched to the limit.
It makes sense to first check with the doctor leading the pregnancy about the position of the fetuses and the location of the placentas according to the latest protocol. If both fetuses lie facing the anterior wall of the uterus, the probability of successful shots is high. If the placenta is located on the anterior wall and blocks the view, it is reasonable to reschedule the visit for one to two weeks: the position of the fetuses often changes during this time.
Before the appointment, it is better to eat and come at a time when children are usually active - a moving fetus is more likely to turn to a convenient angle. The study itself in case of twins takes longer than usual: the doctor works with each child separately, and it takes twice as long to build the volume.
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.