Diagnostic ultrasound ( PDFDrive )

CHAPTER 30 The First Trimester 1049
he goals of irst-trimester sonography include (1) visualization
and localization of the gestational sac (intrauterine or ectopic
pregnancy) and (2) early identiication of embryonic demise
and other forms of nonviable gestation. It also seeks to identify
those pregnancies that are at increased risk for early pregnancy
failure. First-trimester ultrasound accurately dates the duration
or menstrual/gestational age of the pregnancy and assists in early
diagnosis of fetal abnormalities, including identiication of
embryos more likely to be abnormal, based on secondary criteria
(e.g., abnormal yolk sac). In multifetal pregnancies, irst-trimester
ultrasound can be used to determine the number of embryos
and the chorionicity and amnionicity.
Current trends in ultrasound late in the irst trimester focus
on nuchal translucency screening combined with maternal age
and maternal serum screening to determine the risk of chromosomal
abnormalities and structural anomalies. Associated with
the increased emphasis on late irst-trimester ultrasound and
irst-trimester screening, there is an opportunity to visualize
fetal anomalies earlier than at the time of the standard 18- to
20-week scan. First-trimester diagnosis of speciic anomalies is
discussed in the chapters covering those organ systems.
As experience with early irst-trimester ultrasound evolves,
reliable sonographic indicators of ectopic pregnancy and embryonic
demise have been established. 4-6 he accuracy of some
sonographic signs used as indicators of the presence of a live
embryo or of embryonic demise depends on the use of modern,
high-resolution ultrasound equipment and the operator’s expertise.
Published values in the literature based on data using highfrequency
transducers cannot be applied to lower-resolution
5.0-MHz transducers. 7,8 he TVS signs listed in this chapter
assume the use of modern equipment with a transducer frequency
of at least 7 to 8 MHz, with meticulous scanning technique.
Transducers with frequencies of 10 MHz or higher can provide
improved spatial resolution, identifying abnormal and normal
features at even earlier points in pregnancy. 9 Nyberg and Filly 6
emphasize that experienced physicians who interpret ultrasound
rarely rely on a single parameter and simultaneously consider
multiple variables to create a diagnostic impression.
MATERNAL PHYSIOLOGY
AND EMBRYOLOGY
All dates presented in this chapter are in menstrual age or
gestational age, in keeping with the radiologic and obstetric
literature, rather than in embryologic age, as used by embryologists.
his can be counted as follows:
Gestational age = Conceptual age + 2 weeks
Early in the menstrual cycle, the pituitary secretes rising levels
of follicle-stimulating hormone (FSH) and luteinizing hormone
(LH), which cause the growth of 4 to 12 primordial follicles into
primary ovarian follicles 10 (Fig. 30.1). When a luid-illed cavity
or antrum forms in the follicle, it is referred to as a secondary
follicle. he primary oocyte is of to one side of the follicle and
surrounded by follicular cells or the cumulus oophorus. One
follicle becomes dominant, bulges on the surface of the ovary,
and becomes a “mature follicle” or graaian follicle. It continues
to enlarge until ovulation, with the remainder of the follicles
becoming atretic. he developing follicles produce estrogen. he
estrogen level remains relatively low until 4 days before ovulation,
when the dominant or active follicle produces an estrogen surge,
ater which an LH and prostaglandin surge results in ovulation.
Ovulation follows the LH peak within 12 to 24 hours. Actual
expulsion of the oocyte from the mature follicle is aided by
several factors, including the intrafollicular pressure, possibly
contraction of the smooth muscle in the theca externa stimulated
by prostaglandins, and enzymatic digestion of the follicular wall. 11
Ovulation occurs on approximately day 14 of the menstrual
cycle with expulsion of the secondary oocyte from the surface
of the ovary. In women with a menstrual cycle longer than 28
days, this ovulation occurs later, so that the secretory phase of
the menstrual cycle remains at about 14 days. Ater ovulation,
the follicle collapses to form the corpus luteum, which secretes
progesterone and, to a lesser degree, estrogen. If a pregnancy does
not occur, the corpus luteum involutes. In pregnancy, involution
of the corpus luteum is prevented by human chorionic
gonadotropin (hCG), which is produced by the outer layer of
cells of the gestational or chorionic sac (syncytiotrophoblast).
Before ovulation, endometrial proliferation occurs in response
to estrogen secretion (Fig. 30.1). Ater ovulation, the endometrium
becomes thickened, sot, and edematous under the inluence of
progesterone. 12 he glandular epithelium secretes a glycogen-rich
luid. If pregnancy occurs, continued production of progesterone
results in more marked hypertrophic changes in the endometrial
cells and glands to provide nourishment to the blastocyst. hese
hypertrophic changes are referred to as the decidual reaction
and occur as a hormonal response regardless of the site of
implantation, intrauterine or ectopic.
Oocyte transport into the imbriated end of the fallopian
tube occurs at ovulation as the secondary oocyte is expelled with
the follicular luid and is “picked up” by the imbria. he sweeping
movement of the imbria, the currents produced by the action
of the cilia of the mucosal cells, and the gentle peristaltic waves
from contractions of the fallopian musculature all draw the oocyte
into the tube. 13
he mechanism of sperm transport is regulated to maximize
the chance of fertilization and ensure the most rigorous sperm
will be available. 14 From 200 to 600 million sperm and the ejaculate
luid are deposited in the vaginal fornix during intercourse. Sperm
must move through the cervical canal and its mucous plug, up
the endometrial cavity, and down the fallopian tube to meet the
awaiting oocyte within the distal third or ampullary portion of
the fallopian tube. Sperm were thought to move primarily using
their tails, although they travel at 2 to 3 mm per minute, which
would take about 50 minutes to travel the 20 cm to their destination.
Settlage et al. 13 found motile sperm within the ampulla
between 5 and 10 minutes ater deposition near the external
cervical os. If inert particles such as radioactive macroaggregates
or carbon particles are placed near the external os, they too will
be picked up and transported up the uterus and down the tubes.
Contractions of the inner layer of myometrium are believed to
create a negative pressure strong enough to suck up particles
and move them up the endometrial canal. hese contractions