Basic Imaging >
Ultrasound of Early Pregnancy
Section 1: Milestones of Early Pregnancy and US in TM1
Section 2: Natural history of early pregnancy
Section 3: Anatomic Structure of the Early Pregnancy
Section 4: Early Ultrasound Findings in Normal Pregnancy
Section 5: Ultrasound Correlation with Measurement of Human Chorionic
Section 6: Early Pregnancy Failure - Anembryonic Gestation (Blighted Ovum)
Section 7: Bleeding in Early Pregnancy - Threatened Abortion
Section 8: Ultrasound Evaluation of Suspected Ectopic Pregnancy
Section 1: Milestones of
Early Pregnancy and the use of Ultrasound in the First Trimester
during the First Trimester has become an important diagnostic tool to
establish the site and status of the early pregnancy.
Clinical Indications for
First Trimester Obstetric Sonography:
1.) Exclusion of Ectopic pregnancy: Primarily by identification of
2.) Confirm ongoing pregnancy in the setting of vaginal bleeding and pain
3.) Establish gestation age and EDC (due date) when menstrual history
inadequate (First trimester ultrasound gestational age estimates most
4.) In treatment of infertility, early ultrasound confirms intrauterine
location (ectopic location common) and establishes number of embryos (high
incidence of multiple gestation).
Section 2: Natural
history of early pregnancy.
Timing of Early
1.) Menstrual Age: The
traditional duration of pregnancy dates from the first day of the last
Menstrual period an average of 40 weeks to delivery. This period is made up
Pre-ovulatory (follicular) phase of the Ovarian cycle: 13-14 days ending
with ovulation of oocyte from ovary into the peritoneal (fimbriated) end of
the fallopian tube. Variation typically less than 3 days, occasionally
Oocyte migration: The oocyte migrates into the tube, with fertilization in
the tube within 24 hours, typically about day 14.
Fertilization and Zygote migration: The Zygote migrates from tube into the
fundal uterus with implantation on day 22-25.
Implantation: With implantation, trophoblastic HCG production gains access
to maternal circulation and sensitive pregnancy tests turn positive, this
event occurs 3-5 days prior to the first missed period. The local morphology
of the implantation site (by ultrasound imaging) whether in intrauterine or
Growth and Development:
Following implantation, 37 weeks of progressive differentiation and growth
result in the mature fetus.
Early pregnancy loss: About 18% of zygotes do not implant, and about 32% of
implantations spontaneous abort near the onset of the next Menstrual period
(Menstrual Abortion). Only about 1/2 of zygotes persist as a clinical
(symptomatic or noticeable) pregnancy. Compared to this early period, the
later phases of pregnancy result in relatively little additional loss.
The pace of the early
gestational process and the high prevalence of lost and ectopic gestations
create a clinical need to make imaging assessments in the first trimester.
The high resolution, safety, and ease of performance make ultrasound the
procedure of choice.
Section 3: Anatomic
Structure of the Early Pregnancy
The recently implanted
embryonic cell mass forms an interface with the endometrial decidual, the
the trophoblastic cell layer identified as the chorionic layer. The cell
mass actually implants into the substance of the decidua, and the overlying
decidual covers the implanted mass. Because of this the early development of
the gestation appears to occur within the decidual layer, and enlarges to
fill the endometrial cavity later.
Following implantation, a
cavity or sac develops which lies inside the chorionic layer. This
subchorionic layer contain the yolk sack (secondary) and the embryonic disk
(early embryonic cell mass) surrounded by it's own small amniotic cavity.
At about 5 weeks the gestational sac shows a well defined yolk sac within
the chorionic cavity (extraembryonic coelom). The embryo and amniotic cavity
very small. The gestational sac grows at a rate of about 1mm per day. As a
rule of thumb, the menstrual age in days can be estimated by adding 30 days
(first appearence of sac) to the sac size in mm (1mm = 1 day)
As gestation progresses,
the embryo and amniotic cavity grow rapidly and crowd out the chorionic
cavity, and subsequently bulge into and then fill the endometrial (subchorionic)
By high resolution
ultrasound examination these structures can be clearly delineated:
As the Gestation
enlarges, the portion of the chorionic trophoblast destined to form the
placenta enlarges and remains in intimate apposition to the underlying
decidua (decidua basalis). The portion of the chorion on the cavity side of
the gestation expands to form as the decidua capsularis, which comes into
contact with the lining of the free endometrial cavity, now called the
Anatomic Points important
to Ultrasound Interpretation are:
1.) True gestational sacs implant into the endometrial lining, and are seen
eccentric to the endometrial canal. Fluid collections within the canal are
not true gestational sacs.
2.) The Gestational sac and yolk sac (secondary yolk sac) are seen beginning
at 4.5-5 weeks, before a recognizable embryo is seen
3.) As the gestation enlarges into the endometrial cavity, only the early
placenta need be in tight contact with the decidua. Small amounts of
bleeding into the cavity are commonly seen, and may surround much of the
gestational sac, but if the decidua basalis remains intact, the gestation
can and usually does continue to develop normally.
Section 4: Early
Ultrasound Findings in Normal Pregnancy
Ultrasound findings in
the normal First Trimester Pregnancy:
The visualization of
early structures benefits from high resolution technique. In most cases,
ultrasound probes designed to operate in the vagina provide the best
resolution, and a necessary whenever definitive diagnosis cannot be made by
1.) Early Ultrasound appearence: The earliest visible gestational sac is
seen at 4.5 weeks as an echogenic ring, with a tiny central hypoechoic area.
The Nearly horizontal line beneath the sac is the endometrial cavity. Note
the gestational sac lies outside the cavity, embedded in the decidua
(lining). This eccentric position is called the intradecidual sign, seen in
intrauterine implantations, and different from fluid collections in the
endometrial cavity which can be seen in both intrauterine and ectopic
pregnancies. The presence of fluid in the canal in ectopic gestation carries
the risk of mis-identification as an intrauterine pregnancy, and is referred
to as a pseudogestational sac. Pseudosacs never show the intradecidual sign
however. (See Ectopic Pregnancy Section for more information.
2.) Gestational Age Estimate: Measurement of the mean gestational sac
diameter is an effective estimate of gestational age, used between 5 and
5.7-6weeks. The accuracy in this period is about +/-5 days. As soon as an
identifiable embryo crown-rump length (CRL) is measurable (5.7-6 weeks), it
should be used. This is because later gestational sac measurements may not
reflect the embryonic size (or even its presence), but the embryonic CRL
directly reflects embryonic growth. Tables of Mean Sac Size may be used, or
as a rule of thumb, Gestational. Age = 30 + Mean Sac Diam.(mm.)
3.) Yolk Sac : The secondary yolk sac is the first element seen in the
gestational sac. Because it is reliably seen early, usually be 5 weeks, it
is a critical landmark identifying a true gestation sac. Yolk sac should be
seen in normal pregnancy when Mean Sac Diameter is 20 mm by transabdominal
scan, and 8- mm by high resolution vaginal imaging.
It is a spherical membrane, quite echogenic and readily seen.
The embryo is first seen on high resolution scans as a thickening on the
margin of the yolk sac. It may be seen at 2-4 mm Crown-Rump Length (CRL -
Longest Axis) corresponding to 5.7-6.1 weeks gestational age. With
high-resolution, the heartbeat is seen as a regular flutter in the embryo,
first evident at 5mm CRL(6.2 week.). Thus it is possible to see healthy
embryos without heartbeats. In such cases, a follow-up study in 5-7 days
will almost always demonstrate the heartbeat in healthy embryos. The
presence of a heartbeat is a very positive prognostic sign.
By High resolution vaginal scanning, embryos should be seen at Mean Sac
Diameters (MSD) of 18mm, with lower resolution abdominal scanning, embryos
should be seen with MSD of 25mm.
Important Aspects of
Normal Pregnancy Ultrasound:
Early Structures are small and benefit from High Resolution Vaginal Probe
Gestational Sac first appears in the substance of the decidua (intradecidual)
at 4.5 weeks, and should be seen in virtually all normal 5 week intrauterine
The yolk sac is a definite evidence of a true gestational sac, first seen at
5 weeks. It is a landmark to the early embryo, which develops along it's
outer margin . Yolk sac should be seen when sac is 8-10mm. MSD by vaginal
probe, or 20 mm. MSD by abdominal probe.
By vaginal probe high resolution scanning the embryo is first seen between
5.7-6.1 weeks, with heartbeat appearing at 6.2 weeks. Small normal embryos
may not have a heartbeat. Embryo should be seen by High resolution scan at
18mm MSD, or 25 mm MSD by abdominal scan.
Section 5: Ultrasound
Correlation with Measurement of Human Chorionic Gonadotrophin (HCG)
Serum Human Chorionic
Gonadotropin and First Trimester Ultrasound:
measurement of Serum Human Chorionic Gonadotropin has become an integral
part of the evaluation of First Trimester Pregnancies.
When done to qualitatively detect the presence of a pregnancy, highly
sensitive assays allow confident exclusion of pregnancy, even in ectopic
When done quantitatively, HCG values allow rough but useful estimation of
gestational age. Combined with ultrasound findings, HCG measurements allow
systematic evaluation of 1st Trimester complications including pregnancy
loss, ectopic pregnancy, gestational trophoblastic tumors, and ovarian
hyperstimulation associated with Hormonal induction of ovulation.
Because HCG assays have been standardized by at least three standards, the
significance of a given quantitative level (mIU/ml) has changed over the
Between 1964 and 1982, standardization used the Second International
Standard (2nd IS). This standard was less pure and assays based on it are
about 1/2 the values obtained with later standards. Thus if older clinical
studies are used, it is imperative that the standardization be checked.
More recently the WHO First International Reference Preparation and the
Third international Standard have been used, which yield results about twice
as high as the classic (2nd IS) articles on HCG correlation.
Values listed in this section reflect later results unless otherwise
Detection and HCG:
The occurrence of
positive qualitative evidence of pregnancy occurs shortly after implantation
at about 23-28 days (menstrual). The first ultrasound evidence of pregnancy
occurs at about 32-35 days.
In instances where menstrual history is vague, bleeding or pain during this
time may lead to concern for a more advanced ectopic or failing pregnancy.
Since the HCG is positive, and no intrauterine pregnancy is seen, an
erroneous diagnosis of ectopic pregnancy may be made.
To avoid this confusion, it is usual to define a minimum quantitative level
of HCG at which intrauterine pregnancy should be seen by ultrasound.
With transabdominal scanning, this will correspond to about 5 weeks
gestational age, and 3600 mIu/ml (1 IS - Corresponds to 1800 by older 2IRP)
is the usual level chosen. Because the resolution of abdominal scanning is
less than that of vaginal scanning, and much more dependent on body size
(worse in large persons). We now use it only to identify larger and
therefore easier gestations. Whenever an absent or questionable sac is
found, vaginal scanning is always performed.
With high-resolution vaginal scanning, a value of 2000 mIu/ml (1 IS, 3rd IRP)
may be used. Although it is common to see pregnancies at levels below this
level (1000-2000), at these lower levels, confidence is too low to intervene
Using this"discrimination" level, in patients with no high resolution
ultrasound evidence of an intrauterine pregnancy, a level greater than 2000
mIu/ml is considered presumptive evidence of an early ectopic pregnancy.
Patients with similar ultrasound findings and lower HCG levels are
considered indeterminate, and a follow-up HCG and ultrasound are done,
usually in 3-7 days. Follow-up is generally a safe policy, since ectopic
pregnancies with such low HCG values generally do not rupture immediately,
but whenever clinical findings are concerning, laparoscopic evaluation must
Another pitfall occurs
when a recent complete spontaneous abortion has occurred.
If the lost pregnancy is relatively advanced, the starting HCG value may be
many times the discrimination level ( e.g. 8 week pregnancy - HCG =
30,000-100,000 mIu/ml.). In this setting, a substantial serum HCG level may
remain, but ultrasound may show no evidence of the recently lost pregnancy.
These finding may allow a erroneous presumptive diagnosis of ectopic
pregnancy may be considered.
In most cases, the passage of products of conception per Vagina will
identify this situation. In addition less than 10% of spontaneous abortions
result in substantial elevation of HCG. None the less, when the situation
occurs, a decision to perform laparoscopy to exclude ectopic or delay 6-12
hours to follow-up the serum HCG value must be made.
The Following Table lists
the correlation between Serum HCG values as measured by the old Standard
with gestational age and mean gestational sac measurement. Remember to
multiple the HCG values X 2 to use the table with more modern HCG assays.
Section 6: Early
Pregnancy Failure - Anembryonic Gestation (Blighted Ovum)
Development of a normal
appearing gestational sac without and embryo. This likely occurs as a result
of early embryonic death, with continued development of the trophoblast. The
sac may reach considerable size. The empty sac contains no distinguishing
structures. Nearly half of spontaneous abortions occur without an embryo.
When small, the sac cannot be distinguished from the early normal pregnancy.
When larger, it must be distinguished from an endometrial fluid collection
associated with ectopic pregnancy (pseudo-gestational sac), and hemorrhage (subchorionic)
causing false enlargement of a smaller normal sac.
Key Findings in
The sac must be
intradecidual in position, and spherical in shape to be distinguished from
the angular endometrial pseudosac. Detailed, high resolution scanning is
used to distinguish a smaller normal sac complicated by hemorrhage.
The sac must be of sufficient size that the absence of normal embryonic
elements is established.
By High Resolution Vaginal Scanning a sac >13 mm MSD (Mean Sac Diameter)
with no yolk sac is often considered abnormal, but occasional normal
pregnancies do not show yolk sac up to 20mm.
By Lower Resolution Abdominal Scanning a sac >20 mm. MSD with no yolk sac is
abnormal. Vaginal Scanning to improve certainty should then be done.
By High Resolution Vaginal Scanning a sac >18 mm. MSD without an embryo is
often considered abnormal, however normal sacs up to 20 mm. may show no
By Lower Resolution Abdominal Scanning a sac >25 mm. MSD without an embryo
is abnormal. At this sac size, if subsequent Vaginal Scan also negative,
anembryonic nature is virtually assured.
These criteria have been shown to be sufficiently specific to render the
diagnosis of pregnancy failure of the anembryonic type. Since there is no
addition risk, many centers believe it prudent to offer the mother a
follow-up study in 7-10 days exclude error. Such errors are unlikely
In addition to Absent
yolk sac and embryo, several associated finding have been described:
Thin Decidual Reaction (<2 mm.)
Weak Decidual Echo Amplitude
Low Uterine Position
Each of these elements has been shown to be useful, however it is the sac
size and content that are key to this diagnosis. The more subjective nature
of these findings make them subject to bias as well.
Section 7: Bleeding in
Early Pregnancy - Threatened Abortion
Bleeding in the First Trimester.
Vaginal bleeding occurs
during the first 20 weeks in nearly 25% of clinical pregnancies. Since
almost half of these pregnancies will be lost, it is a source of great
concern, and a major indication for ultrasound examination.
Abortion (Embryo Dead):
In many cases, the embryo
will have already died, persistent chorionic function maintains a positive
HCG assay. Expulsion of the sac is often delayed several days, though it may
be seen to slowly migrate from the initial fundal location toward the
The Living Embryo and
The presence of an embryonic heartbeat is highly reassuring. When visualized
by Low Resolution Abdominal sonography, more than 90% of pregnancies
continue. Visualization by high resolution vaginal sonography is associated
with a 70% continuance rate. The apparent discrepancy is because the
heartbeat is a stronger positive sign in the larger more advanced embryos
seen by transabdominal scanning.
The rate of pregnancy loss with positive heartbeat varies with gestational
age and the presence of vaginal bleeding:
-Heartbeat at < 6 week., With bleeding 33% are lost, 16% are lost if no
-Heartbeat at 7-9 week., With bleeding 10% are lost, 5 % are lost without
-Heartbeat at 9-11 week., With bleeding 4 % are lost, 1-2% are lost without
The prognosis for the living embryo improves as gestation proceeds.
in threatened Abortion:
Often visible as endometrial fluid surrounding the external (Decidua
Capsularis) aspect of the gestational sac. As long as the placental (Decidua
Vera) interface of the gestational sac and decidua remain intact, the
pregnancy often continues.
From the standpoint of Hemorrhage volume (Estimated from formula Length (cm)
X Height (cm) X Depth (cm) X 0.52 = Volume ml), less then 75-200 ml. is
often associated with continued development.
Embryonic heart rate < 85 BPM is a negative prognostic sign, but is less
reliable in small embryos.
When the mean sac diameter (MSD) exceeds Crown Rump Length (CRL) by less
then 5 mm., loss rate is 80%, however this "small sac" sign occurs only 2%
of the time.
When Yolk Sac in small
sac is seen (< 13 mm.), and particularly when heartbeat is seen in larger
sacs, expectant management will identify a substantial majority of
successful pregnancies, even in the face of vaginal bleeding.
Section 8: Ultrasound
Evaluation of Suspected Ectopic Pregnancy
Ectopic pregnancy results
from implantation outside the body and fundus of the uterus.
Although ectopic pregnancies occur everywhere from the cervix to the upper
abdomen, 97 percent occur in the fallopian tube.
The clinical importance of ectopic pregnancy stems from the high mortality
of ruptured ectopic pregnancy (0.1%) and the relatively high and increasing
incidence of such pregnancies.
Missed menstrual period
The occurrence of these signs and symptoms is not universal however, and
ectopic pregnancy is potentially serious condition that is notoriously
difficult to exclude clinically.
In recent years, the sensitivity of Serum HCG assays has improved such that
virtually all ectopic pregnancies yield a positive result. Routine HCG
measurement in patients with pelvic pain, vaginal bleeding, and unexplained
adnexal mass allows confident definition of patients at risk for ectopic
The rate of ectopic
pregnancy is increased in patients with tubal disease:
Previous Tubal Reconstruction
Previous Contralateral Ectopic Pregnancy (Nearly 20%)
Previous Tubal Ligation (Pregnancy uncommon 1:1000, but high rate of tubal
Infertility (Increased rate of tubal damage).
There are two important observations in ectopic pregnancy:
1.) Direct visualization of ectopic gestation:
Ideally, ultrasound can
directly localizing the gestation outside the uterus. In practice,
difficulties in visualizing the adnexa and distortion of the gestation sac
mean only 25-60% display unequivocal ectopic gestations. Because of this,
the adnexal findings are usually considered of secondary importance (except
when a definite embryo or yolk sac can be shown). More about adnexal
2.) Absence of
Because the uterus is
reliably seen at high resolution in all patients, any intrauterine pregnancy
of sufficient size will be seen by ultrasound.
The most important single observation in evaluation of suspected ectopic
pregnancy is the presence or absence of an intrauterine pregnancy.
Since the co-existence of both intrauterine and ectopic pregnancy is rare (1
in 4-30,000), in most cases the presence of an intrauterine pregnancy
reduces the probability of ectopic gestation below the level of clinical
None the less, it is good practice to carefully evaluate the adnexa for
alternative causes of pelvic pain, and to identify the rare heterotopic twin
3.) Criteria for
presumptive diagnosis of Ectopic pregnancy:
Positive Serum HCG and absence of detectable intrauterine pregnancy when:
a.) Gestation of 5 week menstrual age or more. Gestations of less then 5
weeks are not reliably seen by ultrasound. Since menstrual age is not 100%
accurate, verification by serum HCG levels above the "discrimination" level
of 2000 mIu/ml. (1 IS, 3rd IRP) suggests that an intrauterine gestation
below the ultrasound detection threshold is present.
Note: In this case, diagnostic laproscopy is often indicated because of a
high probability of ectopic pregnancy. None the less, as many as 35% of
patients with a value of 2000 units and negative ultrasoud will have ongoing
intrauterine pregnancy. Because of this, negative ultrasound and HCG > 2000
units should not be the basis for therapeutic termination, such as
methotrexate injection, but only further diagnostic manueuvers.
b.) No clinical evidence of recent complete spontaneous abortion. In this
setting a small number (about 10%) will have residual serum HCG values
despite recent complete spontaneous loss of an intrauterine pregnancy.
This presumptive diagnosis should lead to Laparoscopic visualization of the
tubes, with appropriate therapy.
4.) In patients with
shock or very severe pain:Ultrasound examination should not be done if it
introduces appreciable delay. In this life-threatening situation, a low-risk
"unnecessary" negative Laparoscopic study is preferable to possible serious
morbidity or death which may result from delayed intervention.
5.) Ultrasound Findings
in Intrauterine Pregnancy:
The structure of gestational sacs is similar in ectopic and intrauterine
At 4.5-5 weeks gestational age, the gestational appears as small hyperechoic
ring implanted into the adjacent tissue or decidua, eccentric to the
endometrial canal. This finding alone is not definitive since occasional
endometrial cysts may give a similar appearence. Early sac often demonstrate
a second ring or decidual cast, constituting the"double decidual cast sign".
These intradecidual sacs are the earliest suggestion of intrauterine
pregnancy, but not 100% diagnostic until yolk sac or embryo appear:
The intradecidual eccentric position is important because fluid collections
in the endometrial canal are common (25%) in ectopic pregnancy. These "pseudogestational
sacs" do not lie in the decidua, and are concentric with the canal. The
pseudosacs below were associated with ectopic gestation.
Adnexal Imaging in
The hyperechoic chorion ring is also the hallmark of small ectopic
gestation, in which the small ring is identified in the adnexa.
More advanced ectopic pregnancies in the 7.5-9 week range may come to lie in
the central area of the pelvis. If the covering tube is thickened, this
ectopic may resemble the uterine fundus, and displace a small true fundus
posteriorly. Be sure to identify the uterine cervix and link it to the
gestation in the fundus in every case. This will avoid mis-identifying a
tubal pregnancy as intrauterine as seen here:
Since ovulation extrudes
the ovum out of the ovarian capsule, ectopic gestation are only rarely
located inside the ovary (<1%). This is important, because corpus luteal
cysts inside the ovary are a physiologic element of all pregnancies. Their
irregular often cystic character may lead to confusion. If a cystic
structure can be localized in the ovary, it is very unlikely to represent an