Time frames and expectations for triplets
Triplets stretch the uterus in the same way as a singleton pregnancy by a month and a half longer. This means that “close” occurs earlier, and volumetric imaging is usually better in the second trimester than in the third. If the goal is to obtain images of children, there is no point in postponing the study until later.
During the appointment, the doctor examines the fetuses one by one, starting with the one who has a pocket of amniotic fluid in front of his face. The fruits of triplets often lie closely together, and a typical situation is when one or two are successfully removed, but the third remains inaccessible. This is a limitation of the method, and not a sign of any trouble.
In addition to images, the volumetric mode is also used for a practical purpose: it clearly shows the relative position of the fetuses and the structure of the partitions between them, which is useful when planning delivery tactics. In this case, diagnostic conclusions are drawn from a conventional two-dimensional study, and the volume serves as a supplement.
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.