Examination of three fetuses at one time
Screening for triplets is the most labor-intensive examination in obstetric ultrasound diagnostics. The full anatomical protocol must be performed three times, and the fetuses almost always partially cover each other, and by the 20th week the uterus is already noticeably crowded. It is realistic to expect that some of the structures in one or two fetuses will remain unobservable and a return visit will be required.
In addition to anatomy, the correspondence of the size of each fetus to its term is assessed. With triplets, growth retardation is the rule, not the exception, so it is not so much the absolute number that is important, but the difference between the fetuses and the dynamics compared to the previous study. Significant discordance requires a transition to a more frequent monitoring schedule with Doppler monitoring.
The length of the cervix in triplets is measured at each study starting from the second trimester, since it is premature birth, and not malformations, that is the main threat in such a pregnancy. The result of this measurement most often determines whether the patient will be observed as an outpatient or hospitalized for prevention.
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.