Mapping three fruits
With triplets, the structural options for placentation are significantly greater than with twins: pregnancy can be trichorionic, dichorionic (when two fetuses share one placenta, and the third has its own) or monochorionic. Each option carries a different level of risk, and it can only be determined in the first trimester, while the septum is clearly visible.
A separate task of this period is mapping. Each fruit is assigned a letter and its position is described in detail: on the right or left, closer to the bottom or to the neck, near the front or back wall. Without such a map, in the next study it is impossible to understand whether the fetus is stunted in growth that was smaller last time, and the entire assessment of the dynamics becomes meaningless. With triplets, this is especially difficult because the fetuses change position and overlap each other.
Research at this time also provides information necessary for family counseling: triplets are considered high-risk pregnancies for preterm birth, growth retardation, and preeclampsia. Early and accurate determination of the structure of placentation allows you to build an observation schedule in advance and determine in which institution to plan delivery.
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.