Congenital malformations - Edocr

334 PART IX MISCELLANEOUS MALFORMATIONS
A six- to tenfold increase in perinatal mortality
rate has been reported in pregnancies associated
with single umbilical artery. 5,9–11 This increase in
perinatal mortality was largely secondary to associated
congenital malformations and intrauterine
growth retardation (IUGR) but an increase in perinatal
mortality has been reported even in infants
with apparent isolated single umbilical artery. 5,11
EMBRYOLOGY
The umbilical cord and its elements are derived
early in embryonic life from the primitive yolk
sac, connecting stalk, and amnion. Initially, two
parallel vascular systems develop from angiogenic
mesenchyme that surrounds the vitelline
duct and the allantoic duct. Two vitelline arteries
and two vitelline veins quickly regress and are
not identifiable by the end of pregnancy. The
umbilical arteries and veins develop from angiogenic
mesenchyme around the allantoic duct.
Initially, a single umbilical artery forms which
subsequently bifurcates in two umbilical arteries.
On the other hand, the umbilical veins are initially
paired structures but, the right umbilical
vein and a portion of the left umbilical vein degenerate
early in gestation and the left umbilical
vein persists as a single umbilical vein during rest
of the gestation. 12
Three mechanisms have been proposed to
explain the embryogenesis of single umbilical
artery: (1) persistence of the original single allantoic
artery of the body stalk, (2) primary agenesis
of one umbilical artery, (3) secondary atrophy or
atresia of a previously normal umbilical artery. 12
Accumulating evidence in the literature strongly
suggests that secondary atrophy or atresia is the
most likely mechanism in a large majority of infants
with single umbilical artery. 1,13,14 Based on
these different mechanisms, four possible types
of single umbilical artery have been described as
follows: 1 (1) type I single umbilical artery is the
most common form that has one umbilical artery
of allantoic derivation and a left umbilical vein;
(2) type II single umbilical artery has one umbilical
artery of vitelline origin and a left umbilical vein.
The umbilical artery frequently originates from the
superior mesenteric artery. This type of SUA is almost
invariably associated with severe fetal malformations
such as sirenomelia, caudal regression,
and anal agenesis; (3) type III single umbilical
artery has one umbilical artery of either allantoic
or vitelline origin and both, the left and an anomalous
persistent right, umbilical veins. This type is
extremely rare and is associated with universally
poor prognosis and fetal malformations; (4) type IV
single umbilical artery has one umbilical artery of
allantoic or vitelline origin and the right umbilical
vein. Only a few cases have been reported to
date and these fetuses were lost early in the
pregnancy.
ASSOCIATED MALFORMATIONS
AND SYNDROMES
The increased rate of congenital malformations
in association with single umbilical artery has
been reported by several studies and ranges from
7% to 65% depending on the differences in the
definition of malformation, methods used for
diagnosis and the reporting practices. 1,8,14–16
These malformations occur in no consistent pattern
and can occur in any organ system. No known
malformation sequence or syndrome is consistently
associated with single umbilical artery. A
study based on birth registry data reported a
fourfold increase in the incidence of major congenital
malformations in babies with two-vessel
umbilical cords (10% for infants with single umbilical
artery versus 2.6% for infants with threevessel
cord). 17 The most prominent associations
(odds ratio >5) in this study were with neural tube
defects, cardiovascular malformations, esophageal
and anorectal atresia, polycystic kidneys, and
limb reduction defects. The mean numbers of
malformations per infant have been reported to
range from 2 to 5. 1 Persutte and Hobbins divided
single umbilical artery associated congenital
malformations into three groups: (1) which
can be identified with prenatal ultrasound;