Diagnostic ultrasound ( PDFDrive )

1054 PART IV Obstetric and Fetal Sonography
menstrual weeks. By the end of the sixth week, blood low is
unidirectional, and by the end of the eighth week, the heart
attains its deinitive form. he peripheral vascular system develops
slightly later and is completed by the end of the tenth week. he
primitive gut forms during week 6. he midgut herniates into
the umbilical cord from week 8 through the end of week 12. he
rectum separates from the urogenital sinus by the end of week
8, and the anal membrane perforates by the end of week 10. he
metanephros, or primitive kidneys, ascend from the pelvis,
starting at approximately week 8, but do not reach their adult
position until week 11. Limbs are formed with separate ingers
and toes. Almost all congenital malformations except abnormalities
of the genitalia originate before or during the embryonic
period. External genitalia are still in a sexless state at the end of
week 10 and do not reach mature fetal form until the end of
week 14.
Early in the fetal period, body growth is rapid and head
growth relatively slower, with the crown-rump length (CRL)
doubling between weeks 11 and 14.
SONOGRAPHIC APPEARANCE OF
NORMAL INTRAUTERINE PREGNANCY
Gestational Sac
Implantation usually occurs in the fundal region of the uterus
between day 20 and day 23. 21 In a study of early implantation
sites in 21 patients, it was found that implantation occurs most
frequently on the uterine wall ipsilateral to the ovulating ovary
and least oten on the contralateral wall. 21 In addition, in a study
of predominant sleeping positions in the peri-implantation period,
Magann et al. 22 found that the 33% of women who slept prone
were most likely to have a high or fundal implantation than
those who slept on their back or side. he latter groups predominantly
had implantations corresponding to their resting
posture.
At 23 days, the entire conceptus measures approximately
0.1 mm in diameter and cannot be imaged by TAS or TVS
techniques. he earliest sonographic sign of an IUP was described
by Yeh et al., 23 who identiied a focal echogenic zone of decidual
thickening at the site of implantation at about 3 1 2 to 4 weeks of
gestational age. his sign is nonspeciic and of limited diagnostic
value.
he irst reliable gray-scale evidence of an IUP is visualization
of a small (1-2 mm luid collection surrounded by an echogenic
rim) gestational sac within the thickened decidua. Yeh et al. 23
originally identiied this sign, referred to as the intradecidual
sign, which is seen at about 4.5 weeks’ gestation. An intradecidual
gestational sac should be eccentrically located within the endometrium.
It is important to ensure that the sac abuts the
endometrial canal to distinguish an intrauterine gestational sac
from a decidual cyst.
he intradecidual sign was originally described on TAS, 23
with a sensitivity of 92%, speciicity 100%, and overall accuracy
of 93% for distinguishing between early IUP and ectopic pregnancy.
Chiang et al. 24 looked at this sign using TVS and found
overall sensitivity of 60% to 68%, speciicity of 97% to 100%,
and overall accuracy of 67% to 73%, indicating that the sign,
when present, is useful for diagnosing an IUP. When absent, it
does not reliably exclude an IUP. It is usually possible to demonstrate
an early IUP as a small intradecidual sac between 4 1 2
and 5 weeks’ gestational age using TVS (Figs. 30.5 and 30.6).
Using a high-frequency (7.5-10 MHz) TVS, Oh et al. 19 were able
to identify a gestational sac in all 67 patients scanned between
28 and 42 days’ gestational age (mean sac diameter [MSD]
between 28 and 35 days was 2.6 mm).
he double-decidual sign (also called double decidual sac
sign) was described by Bradley et al. 25 and Nyberg et al. 26 as a
method for distinguishing between an early IUP and an endometrial
luid collection of other origin, such as the pseudosac
of an ectopic pregnancy. A well-deined double-decidual sign is
an accurate predictor of the presence of an IUP. A vague or
absent double-decidual sign should be considered nondiagnostic
because it does not reliably exclude an IUP. 27
he endometrium in the pregnant state is called the decidua
capsularis, decidua vera, and decidua basalis. he doubledecidual
sign is based on visualization of the gestational sac as
an echogenic ring formed by the decidua capsularis and chorion
laeve eccentrically located within the decidua vera (Fig. 30.6),
forming two echogenic rings. he outer ring is formed by the
echogenic endometrium of the lining of the uterus. he decidua
basalis–chorion frondosum (future placenta) may also be
visualized as an area of eccentric echogenic thickening. he
double-decidual sign was initially described, and is considered
most useful, on TAS. It can usually be identiied by about 5.5 to
6 weeks’ gestational age and is useful in establishing an intrauterine
gestation prior to TAS ability to visualize the yolk sac. It is almost
always resolvable by the time the gestational sac reaches 10 mm,
at which point the yolk sac is typically visible by TVS, thus
diminishing the usefulness of this inding. 28
Parvey et al. 29 found a double-decidual sign in only 53% of
early pregnancies with no yolk sac or embryo present. hey also
assessed visualization of the echogenic chorionic rim alone as
a sign of IUP and found its presence in 64% of cases. It was
more clearly deined in later pregnancies with a higher β-hCG
level (mean, 16,082 mIU/mL) and thin, less clearly deined, or
even absent in the earliest pregnancies. Using a higher-frequency
10-MHz transvaginal transducer to scan patients who had a
positive pregnancy test and only a small (<1 cm) intrauterine
“luid collection” seen with a 6- to 7-MHz transducer, Benacerraf
et al. 9 were able to improve their diagnostic conidence in eight
patients with an IUP. his demonstrates the need to scan TVS
with a high-frequency transducer when an early pregnancy is
in question. Even with use of modern TVS equipment, the
double-decidual sign is absent in at least 35% of intrauterine
gestational sacs. 30
he normal gestational sac is round in the very early stages
and implants immediately beneath the thin, echogenic endometrial
stripe (see Fig. 30.5). As it enlarges, the sac oten has a somewhat
oval shape because of the pressure exerted by the muscular uterine
walls (Fig. 30.7). It can be distorted during TVS by compressing
the uterus with the vaginal probe. he gestational (or chorionic)
sac is illed with extracoelomic or chorionic sac luid, which is
normally weakly relective and more echogenic than the amniotic