Normal Pelvic Anatomy
James M. Shwayder, MD, JD*
KEYWORDS
Pelvic ultrasonography Doppler sonography 3D sonography
Sonohysterography
KEY POINTS
Ultrasonography is the preferred imaging modality for gynecologic disorders.
The vaginal and abdominal imaging approaches are complementary.
Timing of the ultrasonographic study is critical in predicting the findings in reproductive
women.
Determining if organs move during an ultrasonographic examination is critical in predicting
pelvic adhesions.
Video content accompanies this article at http://www.obgyn.theclinics.com.
Gynecologic ultrasonography has emerged as the preferred imaging technique in
women.1 It provides the ability to promptly evaluate patients and correlate one’s clin-
ical findings. The proximity of the pelvic anatomy via the vaginal approach allows the
use of higher-frequency transducers, offering enhanced image resolution. There are
circumstances in which pathology extends out of the pelvis or the pelvic anatomy is
incompletely visualized, necessitating an abdominal imaging approach.2 Thus, one
must not confine the ultrasonographic assessment of the pelvis to one approach,
recognizing that both approaches are complementary.
In premenopausal women, not on ovarian suppression, timing of the study critically
affects the findings at ultrasonography.3,4 Imaging a patient early in the menstrual cy-
cle is considered ideal, as the endometrium is usually thin, less than 5 mm thick, and
ovarian follicles are in a resting state, 10 mm in diameter (Figs. 1 and 2). At midcycle,
at or near ovulation, the endometrium is approximately 10 mm thick and a dominant
follicle approaches 20 to 24 mm in diameter (Figs. 3 and 4). Finally, in the luteal phase,
the endometrium is 15 to 18 mm thick, with a corpus luteum in an ovary4 (Figs. 5 and
6). A basic history, including the patient’s pregnancy history, her last menstrual period,
any hormone use or suppression, and any previous pelvic surgery, can help predict
Disclosure: J.M. Shwayder is a consultant for GE Ultrasound and Siemens Ultrasound.
Shwayder Consulting, LLC, Venice, FL, USA
* 13413 Orino Street, Venice, FL 34293.
E-mail address: jshwayder@hotmail.com
Obstet Gynecol Clin N Am 46 (2019) 563–580
https://doi.org/10.1016/j.ogc.2019.06.001 obgyn.theclinics.com
0889-8545/19/ª 2019 Elsevier Inc. All rights reserved.
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564 Shwayder
Fig. 1. Thin endometrium in the early proliferative phase of the menstrual cycle.
ultrasonographic findings and highlight those that are not consistent with the patient’s
menstrual cycle.
VAGINAL VERSUS ABDOMINAL STUDIES
Vaginal ultrasonography is ideal for evaluation of the normal pelvis, as most target or-
gans are in close proximity to the vagina, regardless of body habitus. This allows com-
plete evaluation in many patients via the vaginal route only. In addition, vaginal
ultrasonography is ideally performed with an empty bladder, which is also desired
for a typical pelvic examination. The higher frequencies in vaginal transducers allows
enhanced detailed imaging, but limits penetration to 10 to 12 cm. Thus, all relevant
anatomy may not be adequately visualized using a vaginal sonographic approach,
for example, nonvisualization of both ovaries, adnexal or ovarian masses rising out
of the pelvis, or when the uterus is enlarged (Figs. 7 and 8). An abdominal approach
may also be required with a uterus oriented in the midsagittal plane. Finally, ureteric
obstruction due to endometriosis warrants evaluation of the kidneys for obstructive
changes, for example, hydronephrosis. As opposed to previous practice, abdominal
ultrasonography does not always require a distended bladder to visualize pertinent
pelvic anatomy, although some patients, for example, pediatric or adolescent pa-
tients, often require a distended bladder for optimal visualization.
Fig. 2. Resting follicles in the early follicular phase of the menstrual cycle.
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Normal Pelvic Anatomy 565
Fig. 3. Midcycle endometrium with trilaminar appearance (arrow).
Fig. 4. Preovulatory follicle (arrow) at midcycle.
Fig. 5. Thickened endometrium is the secretory (luteal) phase.
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566 Shwayder
Fig. 6. Corpus luteum (arrow) during the luteal phase of the menstrual cycle.
Fig. 7. Large fibroid uterus on vaginal ultrasound showing lack of adequate visualization of
the uterus.
Fig. 8. Enlarged fibroid uterus with adequate visualization on abdominal ultrasonography.
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Normal Pelvic Anatomy 567
Fig. 9. Mass in the wall of bladder (arrow) consistent with an invasive endometrioma.
SCANNING PROTOCOL
A scanning protocol should be established such that studies are consistent and com-
plete. This consistent approach to pelvic ultrasonography optimizes detection of sig-
nificant abnormalities. One such approach follows the following sequence:
Vagina (on entry on transvaginal imaging)
Bladder
Uterus and cervix
Endometrium
Right ovary and adnexa
Left ovary and adnexa
Cul-de-sac and uterosacral ligaments
Rectum
Kidneys and ureters
The pertinent ultrasonographic findings and techniques are described in the
following sections.
BLADDER
Sweep from the patient’s right to left, scanning the bladder wall and mucosa. The mu-
cosa should be smooth, although an irregular contour can occasionally be seen when
the bladder is not distended. Masses, such as with an invasive endometrioma, can be
identified during bladder evaluation (Fig. 9). The wall of the bladder is usually
Fig. 10. Measuring the postvoid residual (PVR) in 3 dimensions using 2 orthogonal planes. In
this example the PVR 5 (6.3 2.3 5.8) 0.5233 5 43.9 mL.
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568 Shwayder
Fig. 11. Orientation of the uterus demonstrating an anteverted, anteflexed uterus and a
retroverted, retroflexed uterus.
homogeneous and less than 5 mm thick. A thickness greater than 5 mm can be seen
with chronic cystitis. A postvoid residual can be obtained by measuring in 2 orthogonal
planes and calculating volume using 3 dimensions (Fig. 10). Echogenic debris may
also be seen in the bladder lumen during imaging, and may represent a urinary tract
infection.
UTERUS, INCLUDING CERVIX
Sweep from the patient’s right to left, evaluating the orientation, contour, and size of
the uterus, the characteristics of the myometrium, findings such as fibroids or adeno-
myosis,5 and the morphology of the cervix, such as Nabothian cysts. Both version and
flexion are noted in the orientation (Fig. 11). Version describes the relationship of the
body of the uterus to the vagina, with the cervix to the right of the image with an ante-
verted uterus. Flexion describes the relationship of the body of the uterus to the cervix.
Fig. 12. Sagittal view. Measurements of the uterus in the longitudinal view.
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Normal Pelvic Anatomy 569
Fig. 13. Transverse view. Measurements of the uterus (width) in the transverse view.
The midsagittal plane of the uterus is then evaluated (Fig. 12):
This allows measurement of the length and height of the uterus.
The cervical length can be determined by measuring from the internal os to the
external os.
The thickness of the myometrium anteriorly and posteriorly can be measured.
Significant discrepancies can be found with adenomyosis.6
The myometrial echogenicity is assessed, noting homogeneity or heterogeneity
(as seen with adenomyosis).
The endometrial thickness is measured at the thickest anterior-posterior dimen-
sion in this midsagittal view.
Sweep the uterus in a transverse view, rotating the transducer 90 counterclock-
wise, such that the right side of the uterus is viewed on the left side of the ultrasound
monitor. Rotating the transducer in the counterclockwise manner maintains the same
orientation of the pelvic anatomy and the image on the video monitor. The sweep
traces from the cervix to the uterine fundus and returns to the maximum width of
the uterus. The uterine width is measured at this level (Fig. 13).
Uterine Volume
When calculating the uterine volume, one only measures the length of the uterine
body, excluding the cervix (Fig. 14). The volume is calculated using this uterine length,
the uterine height and width. The equation is [L H W] 0.5233.
Uterine Motion
The uterine sliding sign is demonstrated by visualizing the uterus in the midsagittal
plane and gently advancing the transducer 1 to 2 cm. Normal mobility is demonstrated
Fig. 14. Measurements used to calculate the uterine volume. The length measurement does
NOT include the cervical length.
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570 Shwayder
Fig. 15. Measuring the endometrium when fluid is present.
when the bowel moves along the posterior cervix, posterior uterine body, and over the
uterine fundus (Video 1). The lack of mobility is considered a negative uterine sliding
sign (Video 2) and is associated with obliteration of the cul-de-sac in 93% of cases.7,8
ENDOMETRIUM
The endometrium should be measured in the anterior-posterior measurement at the
widest point, including both layers of the endometrium. In cases where fluid is visual-
ized in the endometrium then each layer is measured individually and added together
(Fig. 15). Evaluate the echogenicity of the endometrium, for example, homogeneous,
heterogeneous, or trilaminar. Note any cystic spaces, irregularities, or possible
masses. Cystic spaces raise concern for an endometrial polyp (Fig. 16). In such cases,
color power angiography can be placed over the endometrium, using a pulse repeti-
tion frequency (PRF) of 0.3 kHz. The presence of Doppler flow within the endometrium
(Fig. 17) suggests the presence of an endometrial polyp, which can be confirmed with
further study, such as sonohysterography (Fig. 18).
RIGHT OVARY AND ADNEXA
There are several technical approaches to identifying the ovaries. Each is replicated on
both the right and left side until adequate visualization is obtained. The approach fol-
lows in a sequential manner until the ovary is visualized:
Fig. 16. Cystic spaces in the endometrium (arrows) suggesting an endometrial polyp.
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Normal Pelvic Anatomy 571
Fig. 17. Doppler flow demonstrated with color power angiography (arrow), consistent with
an endometrial polyp.
Sweep laterally in a sagittal plane.
Sweep the adnexa from the cervix to the fundus with the transducer in a trans-
verse plane.
Start at the fundus in the transverse plane and follow the utero-ovarian ligament
(or approximate location of the ligament) out laterally, then sweeping from the
fundus to the cervix.
Apply gentle abdominal pressure to displace the bowel and bring the ovary into
the imaging field and repeat the above procedures.
On the patient’s right side, rotate the transducer counterclockwise, such that the
top of the transducer is at the 7 o’clock position and rotate it clockwise, identi-
fying the internal iliac vessels, and move the transducer medially. On the patient’s
left side, rotate the transducer clockwise, such that the top of the transducer is at
the 5 o’clock position and rotate it counterclockwise, identifying the internal iliac
vessels, and move the transducer medially. Of note, when evaluating the left
adnexa in this manner, the iliac vessel image will appear as if it is on the right
side of the patient.
If the ovary is not identified with a vaginal approach, one should scan the patient
abdominally (see abdominal protocol), which may lead to ovarian identification or
visualization of an enlarged ovary that is out of the pelvis.
Once identified, the ovary is visualized in 2 orthogonal planes with appropriate
measurements obtained (Fig. 19). A split screen image is used that allows visu-
alization of both planes. The first image is frozen; the transducer is rotated 90
Fig. 18. Endometrial polyp (arrow) from Fig. 17 on sonohysterography.
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572 Shwayder
Fig. 19. Right ovary, using split screen images, measured in 2 orthogonal planes.
toward the pelvic sidewall, which keeps the ovary relatively fixed against the
sidewall.
Mobility of the ovary is demonstrated with the ovarian sliding sign9 (Video 3). In 1
study of patients with previous abdominal surgery, the lack of ovarian mobility
(Video 4) was associated with significant adhesions in 96.3% of patients.
The adnexa is further interrogated to identify the fallopian tube, which is usually
not visualized unless there is fluid in the cul-de-sac (Fig. 20) or the tube is dilated,
for example, a hydrosalpinx (Figs. 21 and 22).
LEFT OVARY AND ADNEXA
The approach to identifying the left ovary is the same as described for the right ovary,
with 1 difference being the rotational sweep in the left adnexa. On the left side, the
transducer is rotated clockwise, such that the top of the transducer is at the 5 o’clock
position to start the rotational sweep. Transducer rotation occurs in the counterclock-
wise manner, until the transducer is in the sagittal orientation. Otherwise, the technical
steps replicate those on the right side.
Fig. 20. Large amount of fluid, associated with malignant ascites, allows visualization of the
fallopian tube (arrow).
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Normal Pelvic Anatomy 573
Fig. 21. Hydrosalpinx on 2D with cystic areas lining up in the adnexa.
Fig. 22. Hydrosalpinx on 3D rendered view with a serpiginous tubular structure of a
hydrosalpinx.
Fig. 23. Small amount of clear fluid (arrow) in the cul-de-sac, which is normal.
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574 Shwayder
Fig. 24. Blood in the cul-de-sac with a ruptured ectopic pregnancy.
CUL-DE-SAC AND UTEROSACRAL LIGAMENTS
The cul-de-sac may have a small amount of fluid present, the amount varying during
the menstrual cycle.10 One study found that 40% of patients have fluid in the cul-de-
sac, with up to a third of patients having fluid present from midcycle to menses. The
fluid may range from clear (Fig. 23) to having low-level echoes, associated with blood
(Fig. 24) or ascites (see Fig. 20). The uterosacral ligaments are usually not visualized
unless there is an abnormal pathologic condition, for example, implants of endometri-
osis.11 Again, mobility of the uterus (see Uterus, above) is critical in evaluating the
presence or absence of cul-de-sac adhesions or obliteration.
RECTUM
The rectum is not routinely interrogated with a normal pelvic ultrasonography. Howev-
er, in cases of deep infiltrating endometriosis (DIE), bowel wall thickening, masses, or
distortion may indicate significant bowel involvement with endometriosis (Fig. 25). The
rectum is carefully evaluated in cases of pelvic floor dysfunction, as seen with tears in
the internal or external sphincter (Figs. 26 and 27).
KIDNEYS AND URETERS
The kidneys and ureters are not routinely visualized with a normal pelvic ultrasonography.
However, with suspected ureteric compression or obstruction, for example, with DIE, one
would be prudent to evaluate the kidney for any signs of retrograde obstruction (Fig. 28).
Fig. 25. Thickened bowel wall in area of deep infiltrating endometriosis (arrow).
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Normal Pelvic Anatomy 575
Fig. 26. Normal rectal sphincter demonstrating the internal anal sphincter (IAS), the
external anal sphincter (EAS), and the puborectalis muscle (PR).
Fig. 27. Abnormal rectal sphincter with significant disruption of the normal continuity of
the sphincter (arrows).
Fig. 28. Significant hydronephrosis (arrow) in a patient with partial obstruction of the right
ureter due to deep infiltrating endometriosis.
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576 Shwayder
Fig. 29. Color Doppler with normal flow spectral analysis showing the peak systolic velocity
(PSV) (solid arrow) and the end diastolic velocity (EDV) (dotted arrow).
Additional Studies
Doppler sonography
It is common to augment 2-dimensional (2D) ultrasonographic studies with Doppler
studies and 3D ultrasonography. This is particularly helpful in determining if a mass
is cystic (lacking central vascular flow) or solid (central Doppler flow identified).
Doppler is applied using 2 primary methods: color Doppler flow and power Doppler.
Color Doppler flow identifies vascular flow toward and away from the transducer.
There is some angle-dependency with color Doppler flow, as well as some interfer-
ence from background noise, particularly in low-velocity states. Color Doppler mea-
sures the frequency shift (eg, velocity) between 2 different points in a vessel. It is
expressed as a spectral analysis, with the Resistance Index (RI), the ratio of the
peak systolic velocity (PSV) minus the end diastolic velocity (EDV) divided by the
PSV, and the Pulsatility Index (PI), the ratio of the PSV minus EDV divided by the
mean of the PSV and EDV. The RI is always a lower number than the PI. Normal resis-
tance to flow is demonstrated with an RI >0.5 and a PI >1.0 (Fig. 29). Low-resistance
flow, signified by an RI <0.5 and a PI <1.0 (Fig. 30), is found with neovascularization,
such as in early pregnancy, with a corpus luteum, and in malignancy. Unfortunately,
there is significant overlap with some malignancies showing normal-resistance flow
parameters, and some normal findings, for example, a corpus luteum, showing low-
resistance flow parameters.
Increasingly, studies are demonstrating that vessel distribution may be more indic-
ative of malignant transformation.12 As such, power Doppler sonography has emerged
Fig. 30. Color Doppler with abnormal spectral analysis showing a low Resistance Index
(RI <0.5) and a low Pulsatility Index (PI <1.0).
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Normal Pelvic Anatomy 577
Fig. 31. Power Doppler sonography in a postmenopausal patient with a granulosa cell tu-
mor. Note the relatively abundant vascular flow within the mass (IOTA 3).
as a preferred alternative in sonographic evaluation of pelvic vasculature. Power
Doppler demonstrates the amplitude of the Doppler signal, rather than velocity, and
is more sensitive than color Doppler, particularly in low-velocity states. By convention,
power Doppler is adjusted to a PRF of 0.3 kHz for evaluation of the adnexa and other
pelvic organs.13 A more recent development is Spectral Doppler, which is directionally
dependent power Doppler sonography. This allows visualization of the amplitude of
Doppler flow, as well as calculating the velocity of flow, particularly helpful in low-
velocity flow states. The International Ovarian Tumor Analysis (IOTA) group has stan-
dardized the description of the flow,13 by assigning a “vascularity score” as follows:
1. No flow
2. Minimal flow
3. Moderate flow
4. Strong flow throughout
A vascularity score of 3 or 4 increases concern for malignancy (Fig. 31). The IOTA
method is covered in Wouter Froyman and Dirk Timmerman’s article, “Methods of
Assessing Ovarian Masses: IOTA Approach,” in this issue.
Three-dimensional sonography
The ability to capture 2D images and manipulate them to demonstrate 3D planes has
dramatically improved the ultrasonographic assessment of various gynecologic findings.
An in-depth discussion of 3D is beyond the scope of this article. A basic understanding of
several modalities is important. The 3D image can be obtained in the longitudinal
(sagittal) plane or the transverse plane (which may be better for some uterine anomalies,
eg, a bicornuate uterus or uterine didelphys). The image representing the plane of acqui-
sition is always displayed in the upper left corner. The perpendicular orthogonal plane is
the upper right corner. The coronal view is in the lower left corner. The rendered view is in
the lower right corner (Fig. 32). Abuhamad and colleagues described a method (the
"Z-technique”) of rapidly and easily manipulating the images to obtain the best coronal
view of the uterus.14 Rotating the C-plane image 90 allows visualization of the coronal
view of a normal uterus in its correct orientation (Fig. 33). An in-depth discussion of 3D
and its ability to diagnose several pelvic abnormalities is covered in Beryl R. Benacerraf’s
article, “Three-Dimensional Volume Imaging in Gynecology,” in this issue.
Sonohysterography is a technique of inserting fluid into the uterus during 2D or 3D
sonography, enhancing the ability to assess the endometrium and evaluate the pres-
ence of any intracavitary focal lesions, for example, endometrial polyps, submucous
myomas, focal hyperplasia, or cancer. A normal uterus will have uniform and
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578 Shwayder
Fig. 32. Uterine image captured in 3D demonstrating (A) the acquisition plane, (B) the
perpendicular orthogonal plane, and (C) the coronal view and the rendered view.
symmetric endometrial echoes, with no focal lesions in the cavity (Fig. 34). This is
covered in more detail in Beryl R. Benacerraf’s article, “Three-Dimensional Volume
Imaging in Gynecology,” in this issue.
Sonosalpingography, so-called hystero-contrast-sonography or HyCoSy, is a tech-
nique to demonstrate tubal patency at the time of pelvic sonography.14 The technique
Fig. 33. Rotation of the coronal view allows correct anatomic orientation of a normal uterus
for the coronal multiplaner view and rendered view.
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Normal Pelvic Anatomy 579
Fig. 34. Fluid inserted into the uterine cavity during sonohysterography demonstrates endo-
metrial echoes that are uniform and symmetric with no focal lesions present. (A) Longitudi-
nal view and (B) transverse view.
involves injecting a contrast agent, either fluid or foam, into the uterus, allowing flow of
the contrast into and out of the fallopian tube, and documenting such flow with ultra-
sonography. This is covered in more detail in Beryl R. Benacerraf’s article, “Three-
Dimensional Volume Imaging in Gynecology,” in this issue.
Pelvic floor ultrasonography is an emerging application using both 2D and 3D ultra-
sonography. It involves multiple approaches including the abdominal approach, a
vaginal approach, a transperineal approach, and a transrectal approach. Application
of ultrasonography to assessment of pelvic floor disorders is increasing exponentially.
This application is discussed in detail in Hadas Allouche Kam and colleagues’ article,
“Ultrasound in Pelvic Floor Dysfunction,” in this issue.
SUPPLEMENTARY DATA
Supplementary data related to this article can be found online at https://doi.org/10.
1016/j.ogc.2019.06.001.
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