Diagnostic ultrasound ( PDFDrive )

CHAPTER 32 Multifetal Pregnancy 1119
TABLE 32.1 Sonographic Findings Used
to Assign Chorionicity and Amnionicity in the
Second and Third Trimesters
Dichorionic
Monochorionic
Diamniotic
Monochorionic
Monoamniotic
Two separate Single placenta Single placenta
placentas or can
merge to appear
as a single
placenta
Twin peak sign T sign —
Different or same Same sex Same sex
fetal sex
Thick membrane Thin membrane No membrane
— — Entangled cords
As a irst step, the placental locations should be determined.
Two separate placentas indicate a dichorionic pregnancy. 1
However, approximately 50% of dichorionic placentas can appear
fused at sonography. 24 If only a single placenta is seen, the fetal
surface of the placenta should be searched for a twin peak or
delta sign, 25-27 indicating a dichorionic gestation. his sign is a
triangular extension of placenta into the intertwin membrane.
Two layers of amnion and two layers of chorion form the intertwin
membrane of a dichorionic gestation, and proliferating chorion
frequently extends between the two chorionic membranes and
separates the amnions, thus forming the peak or triangle (Fig.
32.4). his appearance is not continuous across the entire
membrane, but rather occurs in patches that can be diicult to
identify as the gestational age increases.
In a monochorionic gestation, the separating membrane is
composed of two layers of amnion only, without chorion. he
two-amnion membrane meets the placenta/single chorion in a
T-shaped manner (Fig. 32.5). Because of the presence of four
layers (in dichorionic twins with the two thicker chorions and
two thin amnions) versus two layers of membranes (in monochorionic
twins with two thin amnions) between sacs, the
membrane is subjectively thicker in dichorionic gestations than
in monochorionic gestations, but this method is less reliable for
chorionicity determination than the others described earlier.
When the fetus is large enough to assess anatomy, fetal sex can
assist in diferentiating twin types. Two diferent sexes are obviously
dichorionic, whereas same-sex twins can be either
dichorionic or monochorionic. Monoamniotic twins can be
accurately identiied when there is entanglement of either fetal
parts or the cord (Fig. 32.6, Video 32.3). For higher-order
multiples, any combination of chorionicity and amnionicity is
possible, and more than one means of determining the type of
gestation may be needed (Fig. 32.7, Video 32.4).
GENERAL ISSUES
Multiple gestations require the full sonographic evaluation
appropriate for singleton pregnancies but have additional unique
issues. One important role of the ultrasonologist is to establish
which fetus is which, in a way that will allow continuous identiication
of each fetus until delivery. his determination is made on
the irst ultrasound in the second trimester or in the late irst
trimester at the time of nuchal translucency measurement. he
fetus whose anatomic part is presenting (i.e., is closest to the
cervix) is termed fetus 1 (or A). his fetus should remain fetus
1 for the rest of the pregnancy, even if it moves out of the presenting
position at some point. Although a change in position is
unlikely to happen if the initial determination was correct, it
might occasionally occur, particularly when the intertwin
membrane inserts near the cervix. he nonpresenting fetus is
labeled twin 2 (or B). In larger-order multiples, the location in
the uterus is used as an identifying characteristic—triplet 2 is
upper right, triplet 3 is upper let, or vice versa. here is no
standard method of labeling higher-order births beyond the
presenter as number 1. Whichever method is chosen needs to
be clearly reported and reproducible for all follow-up studies.
In addition to proximity to the cervix, other clues for identifying
which fetus is which are placental location, sex, discordant growth,
and any potential anomaly. For higher-order multiples, the
placentation/chorionicity should be reported at every exam, and
this will help the sonographer with labeling—for example,
dichorionic triamniotic triplets: triplet 1 is presenting with an
anterior placenta, triplets 2 (upper right) and 3 (upper let) share
a posterior placenta.
Good visualization of the intertwin membrane is essential
and can be challenging at later gestational ages and in monochorionic
gestations. he majority of monochorionic pregnancies
are diamniotic, and lack of membrane visualization is typically
due to oligohydramnios or overlapping fetal parts. Assessment
of the amniotic luid volume is performed for each fetus individually.
Fluid volume is assessed either subjectively by an experienced
examiner or using the deepest vertical pocket (see Chapter 42).
Cutofs of greater than 8 cm and less than 2 cm within one sac
are generally accepted values to deine polyhydramnios and
oligohydramnios, respectively. 28 Occasionally, separation of the
intertwin membrane can be appreciated, with amniotic luid
tracking between the membranes. his appearance has not been
shown to be associated with an adverse outcome. 29
he umbilical cord of each fetus should be evaluated for the
presence of three vessels, as both monochorionic and dichorionic
pregnancies have a higher incidence of single umbilical artery
than do singletons. 30 In multiple gestations 17% to 22% have an
abnormal placental cord insertion with its associated adverse
outcomes as compared with 8% of singletons. 31,32 he abnormal
cord insertions include both marginal (11% of twins) and velamentous
(6%) insertions 31 and the placental cord insertion of
each fetus should be documented (Fig. 32.8, Video 32.5). A
velamentous cord insertion in one or both twins is more common
in pregnancies conceived with assisted reproductive technology,
as well as in twins with intrauterine growth restriction. 33-35
Twin pregnancies are known to have higher rates of perinatal
morbidity and mortality than singletons, 36-38 and the risk is closely
related to chorionicity. 39,40 Greater-order multiples have still higher
risks. he primary complications are premature delivery, intrauterine
growth restriction, and intrauterine demise of one or