Screening for triplets is based on ultrasound
With triplets, biochemical screening practically loses its diagnostic value: the concentration of markers depends on the number of placentas, their weight and the individual contribution of each fetus, and there are no reliable correction factors for triplets. Therefore, risk calculation is based primarily on ultrasound markers and the age of the mother.
The thickness of the collar space is measured for each of the three fetuses according to all the rules: strictly in a sagittal section, with a neutral position of the head, with a CTE from 45 to 84 mm. Additionally, the nasal bone, blood flow in the ductus venosus and tricuspid regurgitation are assessed. An increase in TVP in one fetus with normal values in the rest is an important finding that requires separate discussion, since the tactics in such a situation are fundamentally different from the case when all fetuses are affected.
Simultaneously with the markers, chorionicity and amnioticity are necessarily determined and a map of the location of the fetuses is drawn up. This study actually combines two tasks - assessing the risk of anomalies and constructing a surveillance scheme for the entire pregnancy - and therefore takes significantly more time than screening for singleton pregnancies.
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.