Why calculating risk with doubling is more difficult
Biochemical markers of maternal blood - β-hCG and PAPP-A - in twins are produced by two fetuses and two placentas, so their concentration approximately doubles. Standard guidelines are not applicable and the laboratory is required to use correction factors for multiple births. But even with the correction, biochemistry does not allow us to understand which fetus the deviation relates to, and the accuracy of the calculation is noticeably lower than in a singleton pregnancy.
Because of this, with twins, the weight of the ultrasound markers increases. The thickness of the nuchal translucency is measured for each fetus separately, the nasal bone, blood flow in the venous duct and through the tricuspid valve are assessed. For dichorionic twins, the risk is calculated for each fetus individually. With monochorionic fetuses, the fetuses are genetically identical, so the risk is one in two and is usually calculated based on the average TVP value.
Separately, it is worth knowing about the limitations of a non-invasive prenatal test for multiple births: it analyzes a mixture of DNA from both fetuses, and its information content is lower in case of twins, and if an abnormality is detected, it does not indicate which fetus is affected. At high risk, clarification is carried out invasively, with separate sampling of material from each fetus.
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.