Imaging Monochorionic Pregnancies

The MCA peak systolic velocity reflects how fast blood is flowing in the brain. Large differences in the MCA can point to TTTS. The ductus venosus, a unique fetal vessel that funnels a proportion of nutrient-rich umbilical venous return directly into the right atrium, similarly can be used to evaluate cardiac status. Doppler screening of the ductus venosus and MCA has its most useful role early in pregnancy.

Again, because most of the amniotic fluid from 16 weeks on is due to fetal urination, and because changes in urine output reflect changes in blood volume status, the assessment of bladder filling and amniotic fluid volume reveals much about blood volume status and possible TTTS.

Whenever we see a monochorionic twin pregnancy, therefore, we face a range of questions: What are the sizes of the fetuses? Is there a discrepancy? What is the ultrasound end-diastolic velocity in each twin? Is it normal? Or, is there variability in the waveform, which is indicative of hemodynamically significant arterio-arterial anastomoses? Is the amniotic fluid volume normal? What do the bladders look like? Does one fetus have a bladder that's barely filling?

By regularly asking these questions—and using the pregnancy as its own control—we will be alert to the potential problems associated with monochorionicity and more able to proactively plan our monitoring schedules.

A new discrepancy or a change from a previous exam might mean seeing the patient weekly as opposed to every 2 or 3 weeks.

Frequent monitoring is prudent throughout pregnancy as severe TTTS can develop until 22–23 weeks' gestation, even when findings are normal at 18 weeks.

Moreover, milder forms of TTTS, as well as milder forms of selective IUGR, can develop even later.

Umbilical artery Doppler shows significant variation in end-diastolic velocity from positive/absent to markedly reversed, as well as scalloping of the waveform. This indicates the presence of hemodynamically significant arterio-arterial anastomoses.

At left, the presence of chorionic tissue between the layers of amnion from the two sacs produces a “Lambda” sign (circle) that indicates a dichorionic diamniotic pregnancy. At right, the absence of this sign (arrow) indicates monochorionic placentation.

The fetus on the left has a larger abdominal circumference and a higher maximum vertical amniotic fluid pocket, which can point to unequal placenta sharing and/or unequal blood volume and requires follow-up evaluation. Images courtesy Dr. Ahmet A. Baschat

The Complexity of Multiple Gestation

Multiple gestation is an obstetric condition that confronts every obstetrician at some point. Twin pregnancies, for one, are quite frequent, occurring in 3.2% of all pregnancies. Because of this frequency, it is important that we spend some time reviewing the various presentations of twin pregnancies as well as the potential complications.

Twin pregnancies are not a monolithic condition. As we know, twin pregnancies can present in a two-placenta double-membrane sac (dichorionic diamniotic), in a single-placenta double-membrane sac (monochorionic diamniotic), or in some version thereof.

The clinical presentation of twin pregnancies and the potential complications will vary widely, making it of utmost importance to diagnose chorionicity early on. The simple term “twin pregnancy” is not, as our guest author says, a term that is precise enough, in and of itself, to ensure optimal management. A distinction between monochorionic and dichorionic twins must be made.

The complications that are of greatest concern in monochorionic pregnancies involve the anastomoses between the twins' two vasculatures.

In uncomplicated pregnancies, blood is exchanged equally through these anastomoses.

In some pregnancies, however, blood flow becomes unbalanced to the extent that one or both fetuses are compromised.

The management options for complications such as twin-to-twin transfusion syndrome (TTTS) have traditionally been quite limited.

Until recently, management for TTTS involved observation or the removal of excess amniotic fluid.

More recently, however, surgical interventions have been employed with variable success.

Although every obstetrician may not possess the mastery of fetal surgery in these conditions, it is important that all obstetricians nevertheless understand the options that are available and be able to make accurate diagnoses, offer appropriate counseling, and make referrals if appropriate.

Thus, I believe the focus of this Master Class—monochorionicity, its features, and the facets of good management—will be of significant value to the clinician.

We have invited Dr. Ahmet A. Baschat of the department of obstetrics, gynecology, and reproductive sciences at the University of Maryland, Baltimore, to be our guest professor this month. Dr. Baschat is a recognized national expert in fetal therapy, including various intrauterine surgical procedures.

Key Points

▸ Making an accurate diagnosis of chorionicity early in a twin pregnancy is crucial for the prospective management of potential complications. This is because monochorionic twins have unique features that can lead to unequal placenta sharing and unequal blood volume, increasing the risk of fetal death, growth restriction, and other complications. Early in the first trimester is the optimal time to verify chorionicity.