Preoperative Mapping

TTTS. Step Four: Preoperative Mapping

The last aspect of the ultrasound evaluation of patients with TTTS consists of an attempt to predict the location of the dividing membrane and thus the vascular equator and direction of the vascular anastomoses. We call this step preoperative mapping. 

Preoperative mapping may help in choosing the point of entry into the uterus, particularly in patients with a posterior placenta. Mapping is most accurate when the donor twin is stuck, and most difficult in stage I or if a cocoon sign is present. 

In cases where the donor is stuck, preoperative mapping uses the lie of the donor as a guide to the location of the dividing membrane. Indeed, in most cases, the dividing membrane follows the lie of the donor, i.e. longitudinal, transverse, or oblique. Once the lie of the donor is determined, the placental insertion of the umbilical cords is then assessed. 

If the donor twin is between the insertions of the umbilical cords, one might anticipate that there will be anastomoses in the sac of the donor twin and that they will be obscured by this fetus. If, on the other hand, the donor twin is not between the two cords, this fetus should typically not interfere with identification of the anastomoses (Figures 7.15a–c). 

Preoperative mapping is particularly important to avoid unintentional injury to the dividing membrane (unintentional septostomy), which may result in further tearing of the dividing membrane, cord entanglement, and fetal demise.