The intertwin anastomoses account for a range of other pregnancy complications. When placenta sharing is equal but there is a significant mismatch in blood flow and blood volume, twin-to-twin syndrome (TTTS) can develop. In this scenario, the imbalance progresses to the extent that one twin becomes a “donor” of blood volume and the other twin becomes the “recipient.”
A decline in blood volume for the donor twin leads to decreased urine output to the extent that bladder filling virtually ceases and oligohydramnios may progress to anhydramnios. The recipient twin, in the meantime, urinates excessively, leading to polyhydramnios and possibly preterm labor.
TTTS develops in about 10%–15% of monochorionic pregnancies. Overall, however—if you add the approximately 10% that are affected by selective IUGR, and an unknown percentage of pregnancies that may have a bit of both problems or are complicated in other ways to this 10%–15%—I estimate that as many as one-third of monochorionic twins have some kind of significant complication.
For TTTS, endoscopic laser ablation (or laser coagulation) of placental anastomoses has been shown to be an effective treatment and a preferable first-line approach for severe cases diagnosed before 26 weeks. These therapies, however, are available only at specialized centers—a fact that adds to the value of early diagnosis of chorionicity and prospective monitoring for complications.
The Need for Early Diagnosis
We cannot attempt to alleviate complications and improve survival unless a diagnosis of monochorionicity is made early. The diagnosis of chorionicity certainly is more difficult in the second trimester.
However, if a patient has not had a first-trimester scan, a diagnosis should still be attempted.
Monochorionic twin pregnancies remain largely unpredictable. At 12 weeks' gestation, however, if we have diagnosed identical twins, there are several ultrasound parameters we can measure to begin to predict how the pregnancy will proceed and what fetal complications might develop.
Some studies have shown, for instance, that a discrepancy in nuchal translucency between the co-twins of more than 60% means that there is a 60%–70% chance that TTTS will develop.
There also may be some discrepancies in size of other structures that are apparent in the first trimester, such as differences in abdominal circumference, for example, as well as differences in amniotic fluid volume, or bladder size that might be helpful in planning fetal surveillance.
After initial evaluation, we generally recommend that monochorionic twins be evaluated again at 16 weeks, based on research by Dr. Liesbeth Lewi of the University Hospitals in Leuven, Belgium, showing that a combined risk assessment in the first trimester and at 16 weeks can predict selective IUGR or TTTS with greater than 80% accuracy.
In a study of 200 monochorionic diamniotic twin pregnancies, Dr. Lewi found that significant predictors of TTTS, selective IUGR, or intrauterine death in the first trimester were crown-rump length and discordant amniotic fluid volume. At 16 weeks, significant predictors were the differences between the co-twins in abdominal circumference, amniotic fluid volume, and the site of cord insertions. [The site of cord insertion was classified as velamentous, eccentric (more than 2 cm from the placental edge), or marginal (less than 2 cm from the placental edge), and a discordant cord insertion was considered to be the combination of a velamentous cord insertion in one fetus and an eccentric cord insertion in the other fetus.]
The differences between the co-twins in the ultrasound parameters were additive when measured in the first trimester and at 16 weeks. Combined risk assessment detected 58% of the fetal complications by classifying 21% of the 200 pregnancies as high risk, with a false-positive rate of 8%, while the predictive value of one assessment alone was significantly lower (Am. J. Obstet. Gynecol. 2008;199:493.e1–7).
Dr. Lewi's research was among the literature considered recently by a panel of experts assembled by the North American Fetal Therapy Network. The panel has been working on a consensus statement that, when finalized, will make recommendations for early diagnosis of monochorionicity and basic combined risk assessment.
Doppler ultrasound (US) measurements of the umbilical arteries, which depict resistance in the blood vessels and resultant blood flow, also may be helpful. Just as with singleton pregnancies, Doppler US provides information in the monochorionic pregnancy about the vasculature of the placenta and the amount of placenta the fetuses have available for nutrient exchange.
In monochorionic pregnancies, however, Doppler US has the added benefit of being key to diagnosing and evaluating hemodynamically significant arterio-arterial anastomoses that induce variations in diastolic velocity not seen in singleton pregnancies.
The imbalance in blood flow exchange between the co-twins' circulations—again, the primary contributor to the development of TTTS—also can be examined using Doppler assessments of two additional vascular beds: the middle cerebral artery (MCA) and the ductus venosus.