1992-02-16-01 Cord, velamentous insertion © Quintero www.thefetus.net/
Cord, velamentous insertion
Ruben Quintero, MD, Waldo Sepulveda, MD, Roberto Romero, MD *, Francisco Brandt, MD, Moshe Mazor, MD
Definition: Insertion of the umbilical cord on the chorioamniotic membranes rather than on the placental mass.
Types: Insertio velamentosa or inserpositio velamentosa, depending on whether the umbilical arteries branch or not within the membranes, respectively4. A special condition, called vasa previa, occurs when the vessels traverse the fetal membranes below the presenting part.
Prevalence: 1.1% in singletons and 8.7% in twins1.
Pathogenesis: Several theories1,4 have been proposed: 1) abnormal fixation of the yolk sac to the chorion; 2) insertion of the body stalk to a region of proliferating trophoblast other than the decidua basalis; 3) abnormal primary implantation due to obliquity of the embryo during implantation; and 4) trophotropism (see fig. 6).
Associated anomalies: Present in 5.9% to 8.5% of cases10,12. These include esophageal atresia, obstructive uropathies, congenital hip dislocation, asymmetrical head shape, spina bifida, ventricular septal defects, single umbilical artery, bilobate placenta, and trisomy 21.
Differential diagnosis: None.
Prognosis: Increased risk for intrauterine growth retardation, preterm birth, congenital anomalies and fetal bleeding3,6,9-12. Vasa previa: from 75% to 100% fetal mortality rate3,13.
Recurrence risk: Not known to be increased.
Management: Depends on the location of the velamentous vessels. In the lower segment: cesarean section to avoid the risks of vasa previa; above the lower segment: standard obstetrical management. A complete anatomical survey, including fetal echocardiography, and serial ultrasound examinations for fetal growth must be offered.
MESH Placenta-Previa-complications, Fetal-Membranes ICD9 762.6 CDC 762.615
*Address correspondence to Roberto Romero, MD, Dept. of Obstetrics and Gynecology, Wayne State University School of Medicine, Hutzel Hospital, 4707 St. Antoine Blvd., Detroit, MI 48201. Ph: 313-993-2700. Fax: 313- 993-2683. ¶Dept. of Obstetrics and Gynecology, Yale University.
The umbilical cord inserts on the placental mass in about 99% of cases. The insertion site may vary from the center of the fetal surface to the border of the placenta. The term velamentous insertion is used to describe the condition in which the umbilical cord inserts on the chorioamniotic membranes rather than on the placental mass (fig. 1).
Fig. 1: Umbilical cord attachment to the placenta. (from left to right), central; eccentric; and, velamentous insertion. Drawings adapted from reference4.
Therefore, a variable segment of the umbilical vessels runs between the amnion and the chorion, losing the protection of the Wharton"s jelly1-3.
The incidence of this condition is about 1.1% in singleton pregnancies and 8.7% in twin gestations1. In twin pregnancies it is higher in monochorionic placentation or when the placentas are fused than when the placentas are non-fused1. The incidence of velamentous insertion is even higher in early pregnancy; in spontaneous abortions it has been estimated to be 33% between the 9th and 12th weeks and 26% between the 13th and 16th weeks4.
The most significant clinical problem arising from a velamentous insertion of the umbilical cord is vasa previa, a dangerous condition in which the velamentous umbilical vessels traverse the fetal membranes in the lower uterine segment below the presenting part3,5. In 6% of singleton pregnancies with a velamentous insertion, vasa previa is a coexisting condition6. These unprotected vessels may rupture at any time during pregnancy, causing fetal exsanguination and death3. Although spontaneous rupture has been reported before labor and with or without intact membranes3,5,7,8, this accident occurs most often during amniotomy3.
In this communication we present a case report of velamentous insertion of the umbilical cord in which the prenatal diagnosis was made using color flow Doppler ultrasound.
A 32-year-old primigravida woman was referred to our unit at 18 weeks" gestation for ultrasound evaluation because of twin pregnancy and low maternal serum alpha-fetoprotein levels. Gross fetal anatomy, fetal activity, biometry, placental localization, and amniotic fluid volume in both fetuses were normal and consistent with gestational age.
A thick membrane between the sacs and two placental masses were seen, suggesting dichorionic placentation. The umbilical cord of twin A was identified as inserted in the dividing membrane (fig. 2).
Fig. 2: The umbilical cord insertion on the separating membrane (curved arrow).
However, no vessels could be visualized from the site of insertion of the cord to the placental mass with standard bi-dimensional imaging.
By using color Doppler, the abnormal insertion of the cord and the velamentous vessels within the separating membrane were clearly seen (fig. 3 and 4).
Fig. 3: Color flow Doppler ultrasound showing velamentous insertion of the umbilical cord.
Fig. 4: Color flow Doppler ultrasound showing velamentous vessels in the separation membranes.
The frequency of the Doppler signal was consistent with flow from fetal origin (fig. 5), thus confirming the diagnosis of a velamentous insertion of the umbilical cord.
Fig. 5: Doppler signal demonstrating flow from fetal origin.
Because of abnormal alpha-fetoprotein levels, amniocentesis was performed successfully in both sacs, with injection of indigo carmine in the first sac (twin A). Fetal karyotyping was normal in both fetuses.
Embryology & pathogenesis
Although velamentous insertion of the umbilical cord is a frequent finding after delivery, the pathogenesis of this condition is not clearly understood yet.
Several theories1,4 have been postulated to explain this condition (fig. 6): 1) the yolk sac becomes adherent to the chorion at a site which is distant from the definitive placental site; this results in abnormal implantation of the umbilical vessels on the membranes; 2) migration of the body stalk toward a region other than the decidua basalis; this theory proposes that the umbilical cord arises from the chorion which is most richly vascularized.
Fig. 6: Pathogenesis of velamentous insertion of the umbilical cord. Left: abnormal fixation of the yolk sac to the chorion; Middle: insertion of the body stalk to a region of proliferating trophoblast other than the decidua basalis; and Right: abnormal primary implantation due to obliquity of the embryo during implantation. Drawings adapted from reference4.
This is generally the chorion opposed to the decidua basalis, which is the definitive placental site. However, the decidua capsularis may be the area of maximum vascularity in early pregnancy, leading to a velamentous insertion later on; 3) abnormal implantation theory; the blastocyst normally implants with the embryo facing the endometrium. Therefore, oblique orientation of the embryo may result in abnormal implantation of the cord; and 4) trophotropism: the umbilical cord is normally implanted but becomes abnormal because of central atrophy and unidirectional lateral growth of the chorion frondosum.
Prenatal identification of a velamentous insertion of the umbilical cord is a desirable clinical goal since these pregnancies have higher risks for adverse perinatal outcome, i.e., intrauterine growth retardation, preterm birth, congenital anomalies, and fetal bleeding3,6,9-12.
With the introduction of color Doppler technology an accurate prenatal diagnosis of this condition can be easily made. In our case, color Doppler examination was of great help in detecting the velamentous vessels within the separating membranes of a twin pregnancy. Since the separating membrane has no detectable blood flow, the demonstration of flow in this segment of the twin placenta can be considered as a pathognomonic sign of velamentous insertion of the umbilical cord. This finding adds another important rationale for meticulous scanning of all twin pregnancies with color Doppler ultrasound, since velamentous insertion is eight to nine times more common in these pregnancies in comparison to singleton gestations1,13.
Previous cases of velamentous insertion diagnosed prenatally by ultrasound have been reported in association with vasa previa14-16. This report documents, for the first time, the prenatal diagnosis of velamentous insertion of the umbilical cord not associated with vasa previa.
The reported frequency of associated fetal anomalies in fetuses with velamentous insertion varies from 5.9% to 8.5%10,12. These anomalies include esophageal atresia, obstructive uropathies, congenital hip dislocation, asymmetrical head shape, spina bifida, ventricular septal defects, and trisomy 2110. A high rate of deformations instead of disruptions or malformations has been noted12. The possibility of a bilobated placenta1,2 and single umbilical artery must also be considered. Thirteen percent of cases of single umbilical artery are associated with velamentous insertion of the cord17.
Once the prenatal diagnosis of velamentous insertion is made, the following issues should be stressed : 1) to determine the exact localization of the insertion site is critical for proper obstetrical management. If the insertion site is in the lower segment, the risk of having vasa previa is increased. Therefore, a cesarean section must be offered to avoid the serious complications of fetal bleeding. On the other hand, if the insertion site is in the uterine fundus, no changes in the obstetrical management seem to be necessary11. However, the possibility of compression of the velamentous vessels should be seriously considered, and the patient should be followed-up with serial fetal heart rate monitoring in order to detect fetal distress; 2) careful examination of the fetal anatomy, including fetal echocardiography, must be performed to rule out associated anomalies; and 3) serial examinations for fetal growth must be offered, since the incidence of small gestational age infants (defined as birth weight two standard deviations below the mean) with a velamentous insertion of the umbilical cord has been estimated to be 7.5%9. Indeed, the twin with velamentous insertion of the cord has a lower mean birth weight than the unaffected co-twin9.
Color flow Doppler ultrasound is helpful in the prenatal diagnosis and management of cases with velamentous insertion of the umbilical cord.
This work was partially supported by the National Institute of Child Health and Human development.
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