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 jelly^1-3.

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 9^th and 12^th weeks and 26% between the 13^th and 16^th 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 part^3,5. 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 membranes^3,5,7,8, 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.

Case report

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.

Discussion

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 theories^1,4 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.

Diagnosis

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 bleeding^3,6,9-12.

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 gestations^1,13.

Previous cases of velamentous insertion diagnosed prenatally by ultrasound have been reported in association with vasa previa^14-16. This report documents, for the first time, the prenatal diagnosis of velamentous insertion of the umbilical cord not associated with vasa previa.

Associated anomalies

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 placenta^1,2 and single umbilical artery must also be considered. Thirteen percent of cases of single umbilical artery are associated with velamentous insertion of the cord¹⁷.

Management

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.

Conclusion

Color flow Doppler ultrasound is helpful in the prenatal diagnosis and management of cases with velamentous insertion of the umbilical cord.

Acknowledgment

This work was partially supported by the National Institute of Child Health and Human development.

References

1. Benirschke K, Kaufmann P: Pathology of the human placenta.  New York: Springer-Verlag, 1990 pp. 200-4.

2. Fox H: Pathology of the placenta. Philadelphia: W.B. Saunders, 1978, pp. 434-7.

3. Kouyoumdjian A: Velamentous insertion of the umbilical cord. Obstet Gynecol 56:737-42, 1980.

4. Monie IW: Velamentous insertion of the cord in early pregnancy. Am J Obstet Gynecol 93:276-81, 1965.

5. Carp HJA, Mashiach S, Serr DM: Vasa previa: a major complication and its management. Obstet Gynecol 53:273-5, 1979.

6. Paavonen J, Jouttunpaa K, Kangasluoma P, et al: Velamentous insertion of the umbilical cord and vasa previa. Int J Gynaecol Obstet 22:207-11, 1984.

7. Vestermark V, Christensen I, Kay L, et al: Spontaneous intrauterine total rupture of a velamentous umbilical cord: a case report. Eur J Obstet Gynecol Reprod Biol 35:279-81, 1989.

8. Porter CC, Davies NJ, Garden AS: Fetal death due to spontaneous rupture of vasa praevia with intact membranes. J Obstet Gynaecol 11:349-50, 1991.

9. Bjoro K: Vascular anomalies of the umbilical cord. I. Obstetric implications. Early Hum Dev 8:119-27, 1983.

10. Bjoro K: Vascular anomalies of the umbilical cord. II. Perinatal and pediatric implications. Early Hum Dev 8:279-87, 1983.

11. Romero R, Pilu G, Jeanty P, et al: Prenatal diagnosis of congenital anomalies. Norwalk: Appleton & Lange, 1988, pp. 401-2.

12. Robinson LK, Jones KL, Benirschke K: The nature of structural defects associated with velamentous and marginal insertion of the umbilical cord. Am J Obstet Gynecol 146:191-93, 1983.

13. Antoine C, Young BK, Silverman F, et al: Sinusoidal fetal heart rate pattern with vasa previa in twin pregnancy. J Reprod Med 27:295-300, 1982.

14. Gianopoulos J, Carver T, Tomich PG, et al: Diagnosis of vasa previa with ultrasonography. Obstet Gynecol 69:488-91, 1987. 

15. Hurley VA: The antenatal diagnosis of vasa praevia: the role of ultrasound. Aust NZ J Obstet Gynaecol 28:177-9, 1988.

16. Nelson LH, Melone PJ, King M: Diagnosis of vasa previa with transvaginal and color flow Doppler ultrasound. Obstet Gynecol 76:506-9, 1990. 

17.  Heifetz SA: Single umbilical artery. A statistical analysis of 237 autopsy cases and review of the literature. Perspect Pediatr Pathol 8:345-78, 1984.