Controlling Height Differences
After 15 weeks the main issue shifts from chorionicity to growth. With twins, the fruits almost never grow exactly the same, and a slight difference is a variant of the norm. Discordance of more than 20–25 percent of the estimated weight is considered significant: it is associated with an increased risk of an unfavorable outcome and requires more frequent monitoring and sometimes early delivery.
Each study includes complete fetometry of both fetuses, measurement of the maximum vertical pocket of amniotic fluid in each sac separately, a description of the placentas and their location relative to the internal os, as well as the length of the cervix. Cervical shortening in twins is the most important predictor of premature birth, which occurs in approximately half of multiple pregnancies.
The frequency of studies depends on chorionicity. Dichorionic twins are usually monitored every four weeks, monochorionic twins - every two, starting from the 16th week, since feto-fetal transfusion syndrome can develop within a matter of days and is detected precisely by a sharp discrepancy in the volume of water in the fetuses.
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.