The first study determines all tactics
Before 14 weeks, a question is decided on which the entire further pregnancy management plan depends: how many placentas and amniotic sacs the fetus has. The doctor finds the place of attachment of the interfetal septum to the placenta and evaluates its shape. Wedge-shaped thickening - λ-sign - means dichorionic twins: each fetus has its own placenta, the risks are comparable to a singleton pregnancy. A thin septum extending at a right angle - T-sign - means monochorionic twins with one placenta shared and requires monitoring every two weeks from the 16th week.
At the same time, the coccygeal-parietal size of each fetus is measured and cardiac activity is confirmed. A difference in CTE of more than 10 percent already at this time is an early sign that the fruits are developing unevenly. Fetuses are assigned permanent designations A and B with a clear description of their location relative to the cervix and walls; these marks remain throughout pregnancy, otherwise it will be impossible to compare growth over time.
After 16–18 weeks, λ- and T-signs smooth out and chorionicity can no longer be reliably determined. Missing the window means that monochorionic twins will have to be managed blindly as potentially complicated, so this study should not be delayed.
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.