Ultrasound Evaluation during the first Trimester of Pregnancy
Holdorf
SON 2121
OBSTETRICAL SONOGRAPHY I
CHAPTER 6
Normal First Trimester
Fertilization
 On day 14 of the menstrual cycle, the ovum is released and
enters the fallopian tube. Fertilization by the sperm cell
results in formation of a zygote. Important features of
fertilization are as follows:
 A mature ovum is released from the Graafian follicle.
 The ruptured follicle becomes the corpus luteum, which
produces progesterone.
 Progesterone and estrogen stimulate endometrial cellular
proliferation in preparation for implantation.
 Fertilization usually occurs in the ampullary portion of the
fallopian tube within 24-36 hours after ovulation.
 A sperm penetrates the outer layer (Zona
Pellucida) of the ovum.
 Genetic material from the sperm fuses with
the nucleus of the ovum to produce a single
cell (zygote), containing a “set” (23 pairs or 46
total) chromosomes.
 Cell division begins and results in several early
embryonic stages:
 GAMETE The male or female reproductive cell, i.e. the
ovum or spermatozoa
 ZYGOTE the fertilized single-cell organism is called a
zygote prior to mitotic division.
 MORULA The ball of cells, surrounded by the Zona
Pellucida, which is transported through the fallopian tube
toward the uterus.
 BLASTOCYST An organized collection of cells with a cystic
cavity surrounded by trophoblasitc cells; it enters the
uterine cavity about 7 days after fertilization. Trophoblasic
cells differentiate into an inner cell layer (cytotroblaast) and
an outer multicellular layer (syncytotrophoblast). These
cells produce hCG which extends the life of the corpus
luteum. The corpus luteum, in turn, continues to secrete
progesterone which helps assure retention of the
endometrial lining.
Diagram of Fertilization
Fallopian tube demonstrating
fertilization and implantation.
hCG Levels
 Human chorionic gonadotropin (hCG) is a
glyco-protein produced by trophoblasitc
tissue, which is detectable in maternal serum
and urine. It forms the basis of current
pregnancy tests. hCG is believed to support
the corpus luteum, thereby assuring a
continuous supply of progesterone in the first
trimester.
Corpus luteum
not in handouts
 “Yellow body” is a temporary endocrine
structure that is involved in the production of
high levels of progesterone, which in tern
further helps in the release of Gonadotropinreleasing hormone (GnRH) and thus the
secretion of Luteinizing hormone (LH) and
Follicle-stimulating hormone (FSH).
 IN SUMMARY- secretes progesterone, which
is responsible for the decidualization of the
endometrium and its maintenance.
Corpus luteum cyst of
pregnancy
not in handouts
hCG Levels cont…
 First detected 3 weeks after the LMP (7-10
days after ovulation)
 Doubles every 2-3 days
 Plateaus around 8-9 weeks, then declines
 Beta hCG testing can be done two ways
 Qualitative urine results are positive or
negative
 Quantitative results provide specific levels of
the protein present in the blood. The most
common radioimmunoassay method used is
2IRP, although other methods exist. (Second
International Standard 2IS, or Second
International Reference Preparation 2IRP
Quantitative vs. Qualitative
 Quantitative Date
 Deals with numbers
 Data which can be measured
 Length, height, area, etc.
 Qualitative Data
 Deals with descriptions
 Data can be observed but not measured
 Colors, textures, appearance, etc.
hCG levels
 Abnormal levels of maternal serum hCG can
indicate problems with the developing
pregnancy.
 Greater than expected levels are associated with
 Incorrect dates
 Gestational Trophoblasic disease
 Multiple gestations
 Less than expected levels are associated with
 Incorrect dates
 Ectopic pregnancy
 Embryonic demise
Primitive Germ Layers
 In the blastocyst cavity, an inner cell mass is
differentiating from a bilaminar disk to a
trilaminar embryonic disc. There are three
germ cell layers that comprise the embryonic
disk by 5 weeks after LMP:
 Endoderm: inner layer
 Mesoderm: Middle layer
 Ectoderm: outer layer
The three primitive germ
layers
Homework
Not in handouts
 What fetal features arises from the three
germ layers?
 Endoderm
 Mesoderm
 Ectoderm
 From weeks 6 to 10, all major internal and
external structures begin to form. The
primordial heart begins to beat by 6 weeks
GA, but other organ system function remains
minimal.
Implantation
 By the end of the 3rd week LMP, the
blastocyst begins to implant in the
decidualized endometrium. During this
process, as Trophoblastic tissue invades the
endometrium, vaginal bleeding may occur.
This bleeding may be mistaken as a normal
menstrual period.
Placental chorionic Villi
Trophoblastic tissue
 Sonographically, three distinct layers of
decidualized endometrium are observed:
 Decidua capsularis: Closes over and surrounds
the blastocyst
 Decidua Basalis: Develops where the
blastocyst attaches; it contributes to the
maternal portion of the placenta
 Decidua parietalis (decidua Vera) results from
hormonal influence on the uninvolved
endometrial tissue.
 The decidual capsularis and decidua Vera can
be seen sonographically as the Double sac
sign (DSS) of pregnancy. This DSS is not seen
in an ectopic pregnancy, with its
corresponding pseudo-gestational sac.
Double Sac Sign
The DSS double-sac sign
Diagram-DSS
Placenta
 The placenta contains both maternal and
fetal tissue. The maternal component is
derived from the decidua basalis, and the
fetal component is derived from the
Trophoblasic tissue. By 5 weeks LMP, the
trophoblasts develop into chorionic villi.
Those villi in contact with the decidua basalis
rapidly increase in number and vascularity to
become the chorion frondosum, the fetal part
of the placenta.
Membranes of the conceptus
 What sonographers and sonologists refer to
as the chorionic membrane is actually that
chorionic villi which surrounded the
blastocyst but did not further develop into
chorion frondosum. It becomes compressed
and avascular, and appears as a “membrane.”
The chorionic membrane surrounds the
gestational sac and extends up to and merges
with the edge of the placenta.
 The amniotic membrane forms from cells
that originated from the inner blastocyst. It
initially forms opposite the secondary yolk
sac, and is attached to the embryonic disk.
The amnion and its cavity grow rapidly to
surround the embryo, and it actually remains
attached to the embryo at the umbilical cord
insertion site.
 The amnion and the chorion begin to fuse by
the middle of the first trimester. Fusion is
usually complete by 12-16 weeks.
Sonographic identification of the separation
of the two membranes before 16 weeks is a
normal finding and is not associated with a
poor outcome of the pregnancy.
Chorion and amnion fusing
Chorion and Amnion
Early embryo (8 weeks) in
the amnion
Hemodynamic Changes
 Trophoblastic tissue provides the developing
conceptus with nutrients and oxygen. Since
embryonic tissue is highly active metabolically, a
continuous supply of new blood is necessary.
Through the process of angiogenesis and
neovascularizaton, a rich network of small blood
vessels develops to provide necessary perfusion.
The result is high volume, high velocity, and low
resistance flow throughout the cardiac cycle.
This is represented by Doppler spectral
waveforms as high velocity, low resistance flow.
Doppler Spectral waveform
SONOGRAPHIC ANATOMY
Gestational sac
 The identification of a gestational sac (GS) within the
endometrial cavity is the first sonographic evidence
that a normal, intrauterine pregnancy is present. A
GS is ALWAYS seen in a normal IUP when the
following discriminatory levels are achieved:
 Serum Beta hCG > 1,000 mlU/ml (endovaginal using
2IS
 Serum Beta hCG >1800 mlU/ml (transabdominal
using 2IS
 Certain LMP > 5 weeks (with a normal 28 day cycle
Normal sonographic observations - Gestational
sac
 Double sac sign
 Round, oval, well defined
 Echogenic, intact borders
 Positioned in the fundus or mid-uterus
 Grows roughly 1mm / day
 Yolk sac present when MSD is greater than or
equal to 13mm
Mean Gestational Sac Diameter (MSD)
 Mean sac size can be used to date an early
first trimester pregnancy. Because several
extrinsic factors can alter sac dimensions,
MSD is best used prior to the identification of
a crown rump length. The gestational sac
grows at the rate of 1.1 mm per day.
 A mean diameter is calculated from three
planar sections:
 MSD in mm = (AP + Long + Trans) divided by 3
Mean Gestational Sac
Diameter MSD) w/ TV probe
Gestational Sac
Mean Gestational Sac Diameter (MSD)
 Endovaginal measurements are more
accurate:
 MSD – CRL that is less than or equal to 5mm
is associated with a high risk of spontaneous
abortion.
Yolk Sac
 The yolk sac is the first structure seen within
the gestational sac. With endovaginal
ultrasound, the yolk sac can be visible by the
beginning of the 5th week LMP, and almost
always is seen with the MSD is 8mm.
Transabdominally; the yolk sac should be
evident by 7 weeks LMP, when the MSD is
20mm.
 The sonographic appearance of the yolk sac
may be helpful in predicting an abnormal
outcome:
 Diameter > 5.6 mm between 5 and 10 weeks
GA
 Calcified yolk sacs are only seen with
embryonic demise.
normal yolk sac TV
Abnormally Shaped Yolk Sac
Abnormal Yolk sac with Transabdominal
approach
Crown Rump Length (CRL)
 The CRL is the most accurate of all
measurements throughout pregnancy, and is
accurate for dating within 3-5 days if
measured properly. The correct measurement
is obtained from the top of the head to the
bottom of the rump (excluding legs)
Embryonic growth is at the rate of 1mm per
day.
 Rule of thumb: CRL (mm) + 42 days = GA
(days)
CRL measurement
CRL
Embryonic Anatomy
 With
endovaginal imaging, developing
embryonic anatomy is seen much earlier than
with transabdominal imaging alone. Some
structures initially thought to represent
abnormalities have been determined to be
normal in the first trimester.
Rhombencephalon
 The posterior aspect of the fetal crown has a
hypoechoic area which is part of the normal
development of the central nervous system.
This hypoechoic area, the rhombencephalon,
is seen in a midline Sagittal view of the
embryo, and should not be confused with
abnormal dilation of the developing
ventricular system (i.e., Dandy-Walker
malformation or early ventriculomegaly)
the Rhombencephalon
Midgut herniation
 From approximately 9 to 12 weeks of gestation,
the abdominal wall is normally open at the level
of the umbilical cord. This allows herniation of
the intestinal viscera, where it must undergo
midgut rotation before regressing back into the
abdominal cavity. AN echogenic bulge outward
at the base of the umbilical cord is evident, but
this should not be mistaken for a gastroschisis or
an omphalocele. Only after 14 weeks of
gestation should this herniation be diagnosed as
abnormal.
midgut herniation prior to 12 weeks
Homework
1. Explain how FSH and LH influence maternal physiology and the
embryo development?
2. Describe the development of the Placenta and the fetal membranes
(Chorion and Amnion)
3. Describe how to identify the gestational sac: include MSD.
4. Describe the blood flow in early pregnancy.
5. Describe how to identify the embryo and cardiac activity
6. Describe how to determine gestational age in the first trimester.
7. List and describe first trimester complications.
8. What are the risk factors for early pregnancy failure?
9. Describe the yolk-sac evaluation.
10. What are the risk factors that may increase the risk of spontaneous
miscarriage?
11. Explain the usefulness of hCG levels in first trimester pregnancy?
1. The normal gestational sac grows at the approximate rate of

1 mm per day
 2 mm per day
 1.5 cm per week
 2.5 mm per week
2. The corpus luteum cyst of pregnancy is maintained throughout
the first trimester by what process?




Production of luteinizing hormone by the anterior pituitary
Production of hCG by the Trophoblastic cells
Growth of the amnion
Differentiation of the embryonic disk
3. A grown rump length measurement of 16mm
corresponds to what gestational age?




7.6 weeks
58 days
Both a and b
Neither a nor b
4. A thin, echogenic circular structure is seen
surrounding the embryo, separate from the
secondary yolk sac. What does this represent?




Primary yolk sac
Chorionic membrane
Amniotic membrane
Decidua capsularis
5. With endovaginal scanning, a small soft tissue mass is
seen anterior to the abdominal wall of an 11 week
embryo, at the location of the umbilical cord insertion.
What is the probable explanation for this finding?




Early diagnosis of an omphalocele
Early diagnosis of a gastroschisis
Normal herniated bowel in the umbilical cord
Amniotic band syndrome causing evisceration
6. What is the typical Doppler waveform pattern for
trophoblasic flow?




High velocity, high resistance
Low velocity, high resistance
High velocity, low resistance
Low velocity, low resistance
7. A mean sac diameter is used to estimate gestational
age under which of the following circumstances?
 When an accurate CRL cannot be measured
 Prior to visualization of the intracranial landmarks for BPD
 If the sonographic findings suggest an anembryonic
pregnancy
 More than one of the above
8. The chorionic membrane is derived from what
component of the conceptus?




Decidual capsularis
Trophoblasts
Chorionic villi
Decidua basalis
9. A normal yolk sac is first seen with endovaginal
Sonography at approximately what gestational age?




5 weeks LMP
6 weeks LMP
5 weeks after conception
6 weeks after conception
10. In what part of the placenta does maternal blood
circulate?




Chorionic villi
Marginal sinuses
Intervillous spaces
Chorion frondosum
11. During the first trimester, the choroid plexus is seen filling the lateral
ventricles.


True
False
12. The decidua that is associated with the implantation site in the
endometrium is called




Decidua parietalis
Decidua capsularis
Decidua basalis
Decidua Vera
13. False placenta previa can be caused by




Over distended maternal bladder
Lower uterine contractions
Evaluation too early in gestation
All of the above