The evaluation of abdominal pain in the pregnant patient is complicated by the physical changes in pregnancy as well as by the need to avoid fetal radiation exposure. While ultrasound is the preferred initial imaging study, some conditions require alternative imaging. This article presents a review of the use of MRI in the evaluation of abdominal pain in the pregnant patient.
is a Radiology Resident and, at the time this article was
was an Associate Professor of Radiology, University of Texas
Medical Branch, Department of Radiology, Galveston, TX.
is currently an Associate Professor of Radiology and Chief of
Body MRI, Department of Radiology, University of Chicago,
The evaluation of abdominal pain in the pregnant patient can be
a difficult task that is magnified by related sociologic issues and
complicated by the anatomic and physiologic adaptations of the
pregnant state. Evaluation is also limited by the need to avoid
ionizing radiation. Pathologies directly related to pregnancy are
generally evaluated by ultrasound, which usually provides for
accurate assessment of the uterus, fetus, ovaries, kidneys, and
However, ultrasound provides limited and inconsistent imaging of
the bowel, pancreas, and other deep structures and may be rendered
ineffective by overlying bowel gas or anatomic alteration or
variation. In addition, fetal exposure to ionizing radiation limits
the use of computed tomography (CT) during pregnancy.
Serious fetal risk has not been shown to occur with radiation
doses below 10 rad,
which is less than any single commonly ordered diagnostic study.
However, the International Commission on Radiological Protection
has recommended that one should question whether a diagnosis can be
obtained without using ionizing radiation if the expected fetal
dose is high.
The Safety Committee of the Society for Magnetic Resonance Imaging
has issued a report stating that "MR may be used in pregnant women
if other nonionizing forms of diagnostic imaging are inadequate or
if the examination provides important information that would
otherwise require exposure to ionizing radiation."
While fetal harm has not been reported as a result of magnetic
resonance imaging (MRI), the effects of fetal exposure have not yet
been fully determined, which suggests cautious use, especially
during the first trimester. Particular care should be taken with
the use of intravenous contrast, as gadolinium-based agents cross
the placenta. Although adequate controlled studies of gadolinium
use during pregnancy have not been performed, animal studies have
shown an increase in skeletal malformations.
It is suggested that these contrast agents be used only in the
second and third trimester and when potential benefit outweighs the
At our institution, images are obtained with a Signa EXCITE LX
1.5T system (GE Healthcare, Milwaukee, WI) using a phased-array
surface coil when possible. Toward the end of pregnancy, a body
coil may be required. Informed consent is obtained from each
pregnant patient prior to imaging. The backbone of our protocol is
T2-weighted (T2W) imaging in 3 orthogonal planes with single-shot
fast spin-echo (SSFSE) or half-Fourier acquisition single-shot
spin-echo (HASTE) imaging (effective repetition time [TR] =
infinite; echo time [TE] = 80 msec; slice thickness = 6 mm).
Typically, a 35-cm field of view is used, with a 160-192 × 256
matrix. A fat-saturated T2W sequence (SSFSE/ HASTE or fast
spin-echo [FSE]) improves detection of inflammation and
characterization of pelvic masses. An axial T1-weighted (T1W)
sequence is also included, which may be either a breath-hold
spoiled gradient-echo or a respiratory-gated fast spin-echo (FSE)
sequence; TR = 700 msec; TE = minimum; number of excitation (NEX) =
3; echo-train length (ETL) = 2; matrix = 256 × 192, slice thickness
= 6 mm with 1 mm spacing). The FSE sequence provides better
resolution than gradient echo and may be useful for detecting small
structures, such as a normal appendix. After review of the
noncontrast images, the radiologist may request contrast-enhanced
imaging in selected situations. If required, gadolinium may be
administered at a dose of 0.1 mmol/kg, and 2-dimensional or
3-dimensional gradient-refocused echo (GRE) T1W images may
subsequently be acquired.
The evaluation of abdominal pain in the pregnant patient is
complicated by the body's adaptation to the pregnant state.
Physiologic changes include mild leukocytosis, "physiologic"
anemia, mildly elevated alkaline phosphatase, dilatation of the
renal pelvis and ureter, and a relative hypercoagulable state. In
addition, the gravid uterus compresses and displaces abdominal
contents, and abdominal wall laxity during late pregnancy may
diminish signs of peritonitis.
Diagnosis may also be confounded by other conditions common in
pregnancy, such as round ligament pain, constipation, and Braxton
Hicks contractions. This article presents a brief summary of the
use of MRI in the evaluation of abdominal pain in the pregnant
Appendicitis is the most common cause of acute abdomen in the
pregnant patient, affecting in 1 in 1500 pregnancies. Ruptured
appendicitis is associated with a 30% chance of fetal loss, versus
a 3% to 5% risk without rupture.
Moreover, >40% of patients who undergo appendectomy during the
second or third trimesters have a normal appendix, and up to 83% of
all pregnant patients who have an appendectomy experience preterm
(Figure 1). Diagnosis is confounded by "normal" causes of
abdominopelvic pain during pregnancy, physiologic leukocytosis, and
the fact that the gravid uterus may cause the appendix to shift
which distorts the classic clinical presentation. Studies have
proven the ability of MRI to diagnose appendicitis and
differentiate other causes of acute abdominal pain.
In a series of 50 patients, the overall sensitivity, specificity,
and accuracy of MRI for the diagnosis of acute appendicitis in the
pregnant patient were 100%, 93.6%, and 94%, respectively
(Figure 2). The majority of these examinations were obtained
without intravenous contrast.
Small bowel obstruction
Small bowel obstruction is the second most common nonobstetric
surgical indication in pregnancy, complicating approximately 1 in
Adhesions are the cause in 60% to 70% of cases,
with volvulus being the second most common etiology at 25%.
When surgery is required, fetal and maternal mortality rates are
high, approaching 25% and 6%, respectively.
The high mortality may be partly explained by delays and errors
that result from the physical and diagnostic difficulties created
by the pregnant state. MRI provides an effective alternative to CT,
yielding multiplanar imaging of the entire bowel without the
comparatively high radiation dose. Single-shot fast spin-echo
sequences are particularly effective for bowel imaging,
minimizing artifacts caused by magnetic susceptibility and
The third most common nonobstetric surgical emergency in
pregnancy is cholecystitis, occurring in 1 to 8 in 10,000
The lithogenicity of bile is increased during pregnancy; in 1 large
Chilean study, 12% of pregnant women had cholelithiasis versus 1.3%
of nonpregnant women.
While pregnancy has not been shown to result in an increase in
cholecystitis or symptomatic cholelithiasis, cholecystectomy is
associated with miscarriage during the first trimester and
premature labor during the third trimester.
Although signs and symptoms are not significantly altered by
the diagnosis may again be complicated by physiologic changes
related to the state. Ultrasound is obviously the initial study of
choice and is generally accurate. However, if overlying soft
tissues or bowel gas limits evaluation, then MRI may provide
further information. MRI is of particular use if
choledocholithiasis is suspected, as this modality most effectively
reveals findings of biliary obstruction (Figure 3).
Renal stones cause the largest number of nonobstetric
hospitalizations during pregnancy,
with most patients presenting late in gestation. Diagnosis may be
complicated by "normal" dilatation of the renal collecting system,
which results from ureteral compression by the enlarged uterus and
a progesterone-induced decrease in smooth muscle tone within the
urinary tract. "Physiologic" hydronephrosis is usually asymptomatic
but may occasionally result in abdominal pain. While ultrasound
effectively reveals hydronephrosis, it inadequately evaluates the
lower urinary tract and ureteral stones may go undetected. MR
urography is an alternative diagnostic tool. Spencer et al
described physiologic hydronephrosis as compression of the middle
third of the ureter with distal collapse but without an intrinsic
ureteral filling defect. The presence of perirenal fluid, ureteral
dilatation, and an associated ureteral filling defect have been
reported as MRI signs of pathological hydronephrosis, most likely
secondary to a stone.
In these cases, the possibility of clot, an air bubble (resulting
from instrumentation), and flow artifacts must be excluded.
Acute pyelonephritis occurs in 1% to 2% of pregnancies. It is
generally related to asymptomatic bacteriuria or a history of
recurrent lower urinary tract infections.
The clinical presentation is similar to the general population, but
it is associated with preterm labor. Diagnosis is based on
urinalysis, and possible complications are generally shown by
ultrasound. However, if a diagnostic dilemma occurs, MRI may
provide additional evaluation, particularly with regard to the
development of intra- or perirenal abscesses, although the
diagnosis of abscess remains limited without the administration of
While pregnancy does not cause or significantly alter the
clinical presentation of acute pancreatitis, it occurs in
approximately 1 of 3300 gestations, most commonly during the third
trimester and postpartum period.
Ultrasound accurately portrays cholelithiasis and intrahepatic
biliary dilatation but provides limited and inconsistent evaluation
of the common bile duct and pancreas as well as of potential
complications such as pseudocyst. MRI can effectively assess all of
these. Fat-suppressed T1W gradient-echo sequences best evaluate the
pancreas, particularly if pre-and postgadolinium imaging is
obtained. T2-weighted SSFSE imaging accurately shows the biliary
and pancreatic ducts, peripancreatic inflammation, pseudocysts, and
other fluid collections without the administration of intravenous
Inflammatory bowel disease
Inflammatory bowel disease and its complications may be assessed
by MRI, avoiding the comparatively large radiation doses that
result from the usual barium studies and CT. Some reports have
suggested that as many as one third of patients with inflammatory
bowel disease will relapse during pregnancy; this high percentage
may partly be related to a discontinuation of medications.
Maccioni et al
have shown a correlation between disease activity and the intensity
of T2 signal within bowel wall. In addition to identifying bowel
wall inflammation, MRI can effectively show other complications,
such as obstruction, abscess, and possibly fistula formation.
Single-shot fast spin-echo sequences image the bowel ideally,
limiting deleterious artifacts. T2-weighted/T1W imaging with steady
state acquisition (fast imaging employing steady state acquistion
[FIESTA] or fast imaging with steady-state precession [True FISP])
also provides useful information.
Deep venous thrombosis
The risk of deep venous thrombosis is increased by the
hypercoagulable state of pregnancy.
Although ultrasound effectively assesses for deep venous thrombosis
in the lower extremities, evaluation of the pelvic veins is limited
by overlying soft tissue, bowel gas, and the gravid uterus. The
lack of normal flow voids on noncontrast MRI can suggest a deep
venous thrombosis, and time-of-flight MR venography can confirm its
Most pelvic abnormalities (including leiomyomas, ectopic
pregnancy, adnexal torsion, hemorrhagic cysts, and masses) are
initially evaluated by ultrasound, but MRI may be useful when
atypical presentation or ultrasound limitation is encountered.
Uterine leiomyomas are seen in approximately 2% of pregnant women
and 1 in 10 develops related complications during pregnancy,
such as pain resulting from degeneration, rapid growth, or torsion.
The major complication is hemorrhagic infarction that is caused by
venous obstruction at the lesion's periphery, which generally
presents as second- or third-trimester pain, low-grade fever, and
On MRI, degenerating leiomyomas usually have increased T1 and T2
signal, which may be diffuse or peripheral.
A peripheral rim of high T1/low T2 signal may correspond to
If contrast is administered, the entire lesion fails to
MRI may also be of use in unusual or complicated forms of
ectopic pregnancy (abdominal, interstitial, myometrial, or cervical
pregnancy), potentially revealing an associated gestational sac,
mass, hematoma, or hemoperitoneum.
Furthermore, Kataoka et al
have reported that MRI allowed the early diagnosis of ectopic
pregnancy, helping to decide the utility of early conservative
therapy with methotrexate. In addition, adnexal torsion is one of
the few causes of acute abdomen that is more common during
pregnancy, usually occurring between the 6th and 14th weeks.
It accounts for 3% of gynecological emergencies, but 20% to 25% of
cases occur in pregnant women.
When ultrasound fails, as may occur when limited by the gravid
uterus, MRI offers an alternative diagnostic venue, potentially
showing an enlarged ovary, thick and edematous pedicle, signal
compatible with hemorrhage, and smooth wall thickening around a
cystic ovarian mass
(Figure 4). Finally, MRI provides a means of differentiating
hemorrhagic cysts (which have been referred to as "the great
imitator") from a list of differential diagnoses that includes
ectopic pregnancy, adnexal torsion, neoplasm, and pelvic
inflammatory disease (Figure 5). Such cysts may result from rupture
of the corpus luteum, and MRI provides sensitive evaluation for
blood within the cysts or the peritoneal cavity, allowing
differentiation from solid masses.
Diagnosis of abdominal pain in the pregnant patient is fraught
with multiple pitfalls, as discussed above, including a
radiation-induced constraint of most imaging modalities. Ultrasound
remains the initial study of choice, but its limitations
necessitate consideration of alternative technology. While the
evidential role of MRI utilization during pregnancy is currently
being investigated, fetal harm has not been shown, and MRI shows
clear potential to advance patient care.