Double anatomical protocol
Anatomical screening at 18–21 weeks for twins is performed twice - completely and independently for each fetus. The brain with ventricles and cerebellum, face and profile, four-chamber section of the heart with output tracts, lungs, diaphragm, stomach, kidneys, bladder, anterior abdominal wall, spine along its entire length and all segments of the limbs are examined.
In multiple pregnancies, the incidence of congenital defects is higher than in singleton pregnancies, especially in monochorionic twins, where, in addition to the usual defects, there are specific anomalies associated with the common placenta and vascular connections. In addition, only one fetus can have a defect, and this poses separate, rather complex tactical questions for the family and doctors.
The technical complexity of such a study is higher: the fruits overlap each other, and the required cut for one of them is often unavailable. The doctor may ask you to come again to further examine the remaining structures. A mandatory part of the protocol remains the length of the cervix and a separate assessment of the volume of amniotic fluid in each sac.
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.