Placental Cord Insertion

TTTS. Placental Cord Insertion

The site of umbilical cord insertion into the placenta may be described as central, paracentral, marginal, or velamentous. Unlike a central or marginal umbilical cord insertion, which inserts onto the placental disk, the velamentous cord inserts into the fetal membranes (Figure 5.10). 

The bare umbilical vessels, unsupported by either umbilical cord or placental tissue, then traverse the fetal membranes between the amnion and chorion before insertion into the placenta. Although relatively uncommon in singleton pregnancies, the velamentous cord insertion occurs at a significantly higher rate in multiple gestations. 

Kobak et al37 reported the incidence of velamentous insertion of one cord as being nine times higher in twins than the 1–2% found in singleton placentas. In a more recent study of 447 twins,38 the incidence of one velamentous cord insertion in dichorionic twin pregnancies was 6% and in the monochorionic twins it was 18%. Abnormal cord insertions are of concern because of the increased risk of poor perinatal outcomes. Hanley38 reported a 46% rate of birthweight discordance in monochorionic twins complicated by a velamentous cord. Machin39 reviewed 60 consecutive monochorionic twin placentas and noted that the presence of a velamentous cord was associated with higher rates of growth discordance and mortality. 

Of interest, a velamentous cord appeared to be a risk factor for ‘unequal placental parenchymal sharing’. What is the role of a velamentous cord insertion in TTTS? A review of our experience in Tampa has led to several interesting insights. We established a referral system to retrieve the placentas of patients who were prenatally diagnosed and treated for TTTS at our institution. Patients and their referring physicians were asked to ship the fresh placentas after delivery for pathological evaluation. Two hundred and seventy-three complicated monochorionic twin placentas were received between July 1997 and December 2001, of which 249 were TTTS cases. After excluding cases in which the placenta was unsuitable for analysis due to tissue fragmentation or fixation, 168 TTTS cases were available for analysis. 

This was compared with 64 uncomplicated monochorionic twins delivered during the same time period. The incidence of velamentous cord insertion in the TTTS group was 34.5% (58/168). This was significantly higher than the uncomplicated monochorionic twins group, which had a velamentous cord insertion rate of 18.8% (12/64, p = 0.02). Within the groups, our review showed that a velamentous cord insertion was more frequently associated with the smaller fetus. In the TTTS cases, 25.6% (43/168) of the donor fetuses had velamentous insertions vs 10.7% (18/168, p <0.001) of the recipients, whereas in the uncomplicated monochorionic twins group, the rate of velamentous insertion for the smaller fetus was 14.1% (9/64) vs 4.7% (3/64, p = 0.06) for the larger fetus. There were three cases in the TTTS group that had a velamentous cord insertion for both the donor and recipient twins. In a recent study of 76 consecutive TTTS placentas, 

Lopriore et al also found similar results and report that 24% (18/76) of the donors and 3% (2/63) of the recipients had a velamentous cord insertion.40 The excess frequency of velamentous cords in twin placentas complicated by TTTS has led several authors to theorize regarding its role in this syndrome. Fries et al41 noted that one-third of monochorionic twin placentas had such abnormal cord insertions, of which 64% were involved with TTTS. They thus sought an etiological role for the abnormal insertion. Bruner et al42 found a similarly high frequency of velamentous cord insertions of the donor twins, and Mari et al43 also alluded to an etiological role in causing the imbalance that leads to the transfusion syndrome. 

These findings were confirmed by Machin.39 He noted growth discrepancies more commonly in central/velamentous cord insertions of monochorionic twins. Furthermore, AV anastomoses were more common in this situation. These authors’ findings do mirror our own, in that monochorionic twins complicated by TTTS have a higher rate of abnormal cord insertions vs uncomplicated monochorionic twins. However, whether the presence of a velamentous cord insertion serves simply as a risk factor for TTTS or is directly involved in the etiology of the syndrome remains in question. Moreover, several recent studies report a similar incidence of velamentous cord insertion in monochorionic placentas with and without TTTS.42,43,44,45 Does the presence of an abnormal cord insertion impact perinatal outcome in pregnancies complicated by TTTS? From the review of our patients in Tampa, this does not appear to be the case, at least in those that underwent previous laser therapy. 

A velamentous insertion was not associated with the severity of disease as assessed by the Quintero stage. The rate of velamentous cords divided by stage was as follows: stage I, 37.5% (12/32); stage II, 21.7% (10/46); stage III, 40.0% (22/55); stage IV, 41.2% (14/34, p = 0.18). To evaluate the possible influence of a velamentous insertion on perinatal mortality, 23 cases from the TTTS group were excluded from the analysis because they underwent umbilical cord occlusion. Of the remaining 145 TTTS cases, the finding of a velamentous cord did not influence the number of total survivors. In the velamentous cord group, 47.8% (22/46) had two survivors, 30.4% (14/46) had one survivor, and 21.7% (10/46) had no survivors, vs the non-velamentous group, in which 56.6% (56/99) had two survivors, 28.3% (28/99) had one survivor, and 15.2% (15/99) had no survivors (p = 0.52). There were no differences in perinatal outcome if the abnormal cord insertion was present for the donor vs the recipient. Lastly, no differences were found in the birthweight and gestational age at delivery according to type of cord insertion. The proximity of the umbilical cord insertions may influence the caliber of the vascular anastomoses. 

The closer the cord insertions (See chapter 9, Figure 9.11b), the higher the chance of large-caliber anastomoses. The vascular equator may be difficult to define because of the large overlapping vessels, which may have a complex pattern. When the cord insertions are several centimeters apart, the vascular equator is usually well demarcated, with smaller-caliber anastomoses. However, anastomoses caliber and type cannot be predicted solely on cord insertion proximity. An extreme example was illustrated by Wenner,46 who was surprised to find no anastomoses in the placental vascular ramifications of a thoracopagus. 

The cord distance may have important implications regarding perinatal outcome, particularly in those TTTS cases that undergo laser treatment. In a review of 226 patients that underwent laser therapy for TTTS at our institution, we noted that cord distance may be related to perinatal survival. The cord distance was significantly greater in those cases with at least one survivor (14.6 ± 0.90 cm) vs pregnancies with no survivors (11.75 ± 5.43 cm, p = 0.036). In this study, we were not able to identify a relationship between the cord distance and the number of communicating vessels.