892
Obstetrical Complications
SECTION 11
be threefold for twins and 24-fold for triplets or quadruplets.
Finally, these mothers are at increased risk for depression compared with women with a singleton pregnancy (Choi, 2009).
twinning observed in pregnancies conceived in this manner
(Wenstrom, 1993).
The outcome of the monozygotic twinning process depends
on when division occurs. If zygotes divide within the first
72 hours after fertilization, two embryos, two amnions, and
two chorions develop, and a diamnionic, dichorionic twin
pregnancy evolves (Fig. 45-1). Two distinct placentas or a
single, fused placenta may develop. If division occurs between
the fourth and eighth day, a diamnionic, monochorionic twin
pregnancy results. By approximately 8 days after fertilization,
the chorion and the amnion have already differentiated, and
division results in two embryos within a common amnionic
sac, that is, a monoamnionic, monochorionic twin pregnancy.
Conjoined twins result if twinning is initiated later.
It has long been accepted that monochorionicity incontrovertibly indicated monozygosity. Rarely, however, monochorionic twins may in fact be dizygotic (Hack, 2009). Mechanisms
for this are speculative, but Ekelund and coworkers (2008)
found in their review of 14 such cases that nearly all have been
conceived after ART procedures.
MECHANISMS OF MULTIFETAL GESTATIONS
Superfetation and Superfecundation
TABLE 45-1. Selected Outcomes in Singleton and Twin
Pregnancies Delivered at Parkland Hospital
from 2002 through 2012
Outcome
Pregnancies
Birthsa
Stillbirths
Neonatal deaths
Perinatal deaths
Very low birthweight
(< 1500 g)
Singletons (No.)
78,879
78,879
406
253
659
895
(5.1)
(3.2)
(8.4)
(1.0)
Twins (No.)
850
1700
24
38
62
196
(14.1)
(22.4)
(36.5)
(11.6)
a
Birth data are represented as number (per 1000).
Data courtesy of Dr. Don McIntire.
Twin fetuses usually result from fertilization of two separate
ova–dizygoticc or fraternal twins. Less often, twins arise from
a single fertilized ovum that divides–monozygoticc or identical
twins. Either or both processes may be involved in the formation of higher numbers. Quadruplets, for example, may arise
from as few as one to as many as four ova.
■ Dizygotic versus Monozygotic Twinning
Dizygotic twins are not in a strict sense true twins because they
result from the maturation and fertilization of two ova during
a single ovulatory cycle. Moreover, from a genetic perspective,
dizygotic twins are like any other pair of siblings. On the other
hand, monozygotic or identical twins, although they have virtually the same genetic heritage, are usually not identical.
As discussed subsequently, the division of one fertilized
zygote into two does not necessarily result in equal sharing of
protoplasmic material. Monozygotic twins may actually be discordant for genetic mutations because of a postzygotic mutation, or may have the same genetic disease but with marked
variability in expression. In female fetuses, skewed lyonization
can produce differential expression of X-linked traits or diseases.
Furthermore, the process of monozygotic twinning is in a sense
a teratogenic event, and monozygotic twins have an increased
incidence of often discordant malformations (Glinianaia, 2008).
For example, in a study of 926 monozygotic twins, Pettit (2013)
reported a 12-fold increase in the prevalence of congenital heart
defects, but 68 percent of affected infants had a normal sibling.
Accordingly, dizygotic or fraternal twins of the same sex may
appear more nearly identical at birth than monozygotic twins.
Genesis of Monozygotic Twins
The developmental mechanisms underlying monozygotic twinning are poorly understood. Minor trauma to
the blastocyst during assisted reproductive technology
(ART) may lead to the increased incidence of monozygotic
In superfetation, an interval as long as or longer than a menstrual
cycle intervenes between fertilizations. Superfetation requires
ovulation and fertilization during the course of an established
pregnancy, which is theoretically possible until the uterine cavity
is obliterated by fusion of the decidua capsularis to the decidua
parietalis. Although known to occur in mares, superfetation is
not known to occur spontaneously in humans. Lantieri and colleagues (2010) reported a case after ovarian hyperstimulation
and intrauterine insemination in the presence of an undiagnosed
tubal pregnancy. Most authorities believe that alleged cases of
human superfetation result from markedly unequal growth and
development of twin fetuses with the same gestational age.
Superfecundation refers to fertilization of two ova within the
same menstrual cycle but not at the same coitus, nor necessarily
by sperm from the same male. An instance of superfecundation
or heteropaternity, documented by Harris (1982), is demonstrated in Figure 45-2. The mother was sexually assaulted on
the 10th day of her menstrual cycle and had intercourse 1 week
later with her husband. She was delivered of a black neonate
whose blood type was A and a white neonate whose blood type
was O. The blood type of the mother and her husband was O.
■ Frequency of Twinning
Dizygotic twinning is much more common than monozygous
splitting of a single oocyte, and its incidence is influenced by
race, heredity, maternal age, parity, and, especially, fertility
treatment. By contrast, the frequency of monozygotic twin
births is relatively constant worldwide—approximately one set
per 250 births, and this incidence is generally independent of
race, heredity, age, and parity. One exception is that zygotic
splitting is increased following ART (Aston, 2008).
The “Vanishing Twin”
The incidence of twins in the first trimester is much greater than
the incidence of twins at birth. Studies in which fetuses were
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