Ultrasound Basics in Obstetrics
Nancy Nguyen MS3OHSU Diagnostic Radiology Elective
• Review basic female anatomy• Orientation to ultrasound use in OB• Review of OB cases to show how ultrasound
can be used in OB
Female Anatomy Review:
Lets get oriented with what we are working with….
transabdominal ultrasound vs.
Transabdominal Ultrasound • Transabdominal approach :
– Lower frequency, lower resolution image – Curved linear transducer– Better visualized with full bladder
• Can see coronal and sagittal views oforgans and fetus
Indicator on side of transducer
Transvaginal Ultrasound • Transvaginal approach:
– Higher frequency, higher resolution image– Endocavitory probe– Better visualized with empty bladder– Can see sagittal or coronal view of uterus– RULE OF THUMB: if possible attempt transabdominal before considering transvaginal to avoid more
Fundus of uterus cervix
Now to ultrasound with a fetus inside!
Try to have a system when scanning transabdominally:• Start at suprapubic area with indicator pointing to patient’s 9 o’clockprovides a
conventional coronal image with left side of monitor screen as patient’s positional right
- Move transducer cranially this will allow you to see coronal sections of entire uterus & fetus
• Now change indicator to point at 12 o’clock provides conventional sagittal image with left side of screen as patient’s cranial end
- This will allow you to see sagittal sections of fetus corornal view sagittal view
• Now that we are well oriented… lets see what else we can see in a pregnancy…
But first, lets get some abbreviations out of the way:- GA: gestational age- EDD: estimated due date- LMP: last menstrual period- CRL: crown rump length- EFW: estimated fetal weight- US: ultrasound- G’s and P’s: G1P1 one pregnancy (G) and one living child (P)
Confirm viable pregnancy: • Gestational Sac (GS):
– Visible at 4-5wks GA with transvaginal US– Visible at 6 wks GA with transabdominal US– echogenic ring with anechoic center within uterine cavity– Measure by Mean Sac Diameter: average dimensions of width/length/height of sac – GS size increases by about 1mm/day in early pregnancy– Discriminatory zone: serum hCG level in which gestational sac is expected to be visible by US : hCG >2000 mIU/ml
First trimesterConfirm viable pregnancy:Yolk Sac: bright ring with anechoic center located inside GS seen at 5wk GA.
Fetal Pole: represents fetal development at somite stage. Can be seen by transvaginal US as thickening of yolk at 6wks GA.
Fetal heart beat : usually seen around the time fetal pole is present, further confirming viability
First TrimesterMeasuring Gestational Age:• crown rump length (CRL)
– Approximately estimates GA from 7-12wks gestation – Measure longest length of embryo excluding limbs or yolk sac– A Rule of thumb of estimating GA: 6wks + CRL(mm) = 6wks+daysEstimating due date: – For 1st trimester if GA measures within 7days of EDD by LMP then do not change EDD– For 2nd trimester if GA measures within 10days of EDD by LMP then do not change EDD– If ultrasound provides EDD more/less than the 7 or 10 days, then EDD is changed to ultrasound EDD – Once GA confirmed with first trimester CRL, EDD should NOT be changed in further CRL
Other measurement parameters used to estimate gestational age• Biparietal diameter• Femur length• Abdominal circumference
The various parameters can be used in a specific equation providing estimated fetal weight (EFW)
First Trimester:Thickened Nuchal Tanslucency (NT):• One of the parameters used in sequential screening (SS) for Down’s syndrome in first
trimester– SS: Pregnancy associated plasma protein levels, hCG levels, NT thickness
• Measured during 11-14 wks gestational age• Seen on sagittal image as increased subcutaneous non-septated fluid in posterior fetal neck• Measurement >3mm usually considered abnormal, however exact cut off measurements are
dependent on maternal age/gestational age• Detection rate of screening for Down’s Syndrome in first trimester:
– sequential screening with NT: 82-87% – NT alone: 64-70%
Now lets try some cases..
Case 1A 23 year old G1P0 comes in to the clinic to confirm her pregnancy status. Based on her
last menstrual period (LMP) she is 8 wks 2days pregnant. She took a home pregnancy test yesterday which was positive. To confirm her pregnancy you do the following:
• Repeat urine hCG test: Positive• Transvaginal ultrasound:
US findings: Gestational sac and CRL measuring at 7wks gestational age - There was a detectable heartbeat
Question: Is this a normal pregnancy?
Case 1Question: Is this a normal pregnancy? YES!
Confirmed viability by ultrasound:- Presence of gestational sac- Presence of fetal pole with CRL 7wks- Presence of fetal heart beat
Bonus Questions:-So what explains the difference between the GA from estimated LMP and the
estimated GA with ultrasound?
- Which EDD should be used as the more accurate due date?
Answer to bonus questionsSo what explains the difference between the estimated LMP and the estimated GA
with ultrasound?– Many patients may not remember accurate date of LMP. Most likely discrepancy is due
to miscalculation of original EDD based on last menstrual period.
Which EDD should be used as the more accurate due date?– Estimated EDD by LMP was 8wks 2days while ultrasound estimates 7 wks. – Discrepancy in dating is in the first trimester that is more than 7 days apart – Thus gestational age via ultrasound of 7wks should be used with corresponding EDD
Case 2A 28 y/o G1P0 comes in for her first prenatal visit. Patient has been reliably tacking her menstrual cycle for the past year.
Based on her LMP, her estimated EDD suggests she is 9 wks pregnant. She reports pregnancy has been uncomplicated. Upon ultrasound you see:
Findings:- Echogenic getational sac with in uterine cavity, GS measuring 5wks
Question: Is this a normal ultrasound finding?
Case 2Question: Is this a normal ultrasound finding? NO!!! - This case is suggestive of a Missed Spontaneous Abortion with a non-viable gestational sac- At 9wks GA expected ultrasound findings include:
- yolk sac, embryo, fetal heart beat- CRL of embryo measuring close to 8wks GA
Spontaneous abortions: - Should be evaluated by transvaginal ultrasound
- diagnosed by ultrasound within 20wks of pregnancy - often not associated with any specific symptoms besides possible first trimester vaginal bleeding - Occur in 15-20% of first trimester pregnancies, 80% of which are during first 12wks pregnancy
BONUS QUESTION: What is the most likely etiology of the spontaneous abortion?
Answer to Bonus• Approximately 50% of early 1st trimester spontaneous abortions are attributed to
chromosomal abnormalities. Most common is a non-viable trisomy.
• In comparison, 2nd trimester abortions are less likely due to chromosomal abnormalities.
Case 3A 24 y/o female G0P0 comes in to the ED with acute onset of right lower quadrant abdominal
pain that started late last night. She is sexually active and unclear of LMP . She reports that she had vaginal spotting this week which is unusual because she usually does not spot between periods. Sexual history is significant for h/o chlamydia/gonorrhea 2 years ago that was appropriately treated with antibiotics.
Physical Exam: She is afebrile, tender to palpation to RLQ with palpable right adnexal mass.
What initial test should be done in the ED?- Pelvic ultrasound imaging - Urine hCG levels
Case 3 RESULTS:- Elevated urine hCG levels suggestive of pregnancy.- Transabdominal ultrasound of right adnexa - Transvaginal ultrasound of uterus
US Findings:- Trans abdominal US shows echogenic gestational sac with presumable yolk sac - Gestational sac NOT surrounded by uterine tissue- Transvaginal US shows empty urterine cavity
Question: Is this most likely just a regular intrauterine pregnancy?
Case 3Question: Is this most likely a regular intrauterine pregnancy?NO! This case is most likely a Tubal Ectopic Pregnancy!
Common presentation of tubal ectopic pregnancy:- Women of child bearing age- Amenorrhea- Vaginal bleeding- Acute lower quadrant pain
Further workup: - In normal intrauterine pregnancy, serum hCG levels should increase about 60% in 48hrs- Doing a 48hr serum hCG test that shows <60% increase may further suggest abnormal
BONUS Question: Does this patient have any risk factors for an ectopic pregnancy?
Answer to Bonus:
• Patient’s h/o of chlamydia/gonorrhea puts her at increase risk of developing tubal ectopic pregnancy. This is found to be especially true if past infection was an ascending infection that caused inflammation of fallopian tubes that resolved with scarring of fallopian tube. This may increase risk of fertilized egg getting stuck in tube.
• Common risk factors for tubal ectopic pregnancy includes:– h/o chlamydia/gonorrhea– h/o of pelvic inflammatory disease– h/o of tubal ligation
Lets get back to some more normal ultrasound findings in pregnancy…
Placenta Attachment• Ultrasound can be used to determine position of placenta attachment in the
uterine cavity. This information may help in management of delivery during labor.• Placenta can be seen attached to any segment of uterine cavity. • Placenta is seen as hyperechoic thickening of uterine cavity. Some examples:
Anterior placentaPosterior placenta
Case 432 y/o G1P0 at 30 wks GA comes in to the ED with complaints of 1-2 hrs of vaginal
bright red bleeding that started today after having sex with her husband.- She denies pain, uterine contraction, leakage of fluid or trauma. - On exam, uterine tone and fetal heart tones are normal. - She is visiting from out of town and does not have any of the prenatal records
• You are the magnificent medical student who decides to do a transabdominal ultrasound and you see the following:
QUESTION:Does the imaging help explain the patient’ssymptoms?
Case 4Does the imaging help explain the patient’s symptoms?• Yes! The ultrasound shows an example of PLACENTA PREVIA : placenta attachment
completely covering the internal os of the uterine cervix– Normally lower placental edge should be at least 2 cm from the margin of the internal cervical os. – When seen in early pregnancy, it is expected to resolve as placenta often transmigrates away from
internal os as uterine expands through out pregnancy– Often presents as painless bleeding in 2nd or 3rd trimester for <2hrs duration– Medical management: Can often be observed after first episode bleeding, multiple episodes may
necessitate delivery – Unresolved placenta previa will deliver by c-section
There is a spectrum of placenta previa:
Case 4 Bonus Questions:• What would you be concerned of if the patient had presented with 3rd trimester
vaginal bleeding that was PAINFUL with irregular uterine contractions?
• What would you expect to see on ultrasound imaging?
• What would be the medical management compared to a situation of placenta previa?
Answers to bonus questions• You would be concerned with PLACENTA ABRUPTION: premature separation of placenta from endometrium.
– Commonly presents as painful vaginal bleeding with irregular contractions– Bleeding is continuous once starts and is most common cause of coagulopathy in pregnancy
• Ultrasound findings:
• Medical management: observe vitals, fluid administration, emergency c-sections in cases of severe hemorrhageTake home point: top differential for 3rd trimester vaginal bleeding:
Placenta Previa vs. Placenta Abruption
Both can be confirmed with ultrasound
Hemorrhage from separation of placenta from endometrium
• Now just for fun lets do one more case…
Case 525 y/o G1P0 at 14wks GA dated by LMP. She recently migrated from Vietnam and
comes in for her first prenatal care, she did not have any first trimester prenatal care. Pregnancy was confirmed by multiple urine pregnancy test 7 wks ago.
- Pregnancy has been complicated with recent vaginal bleeding with out pain. - You notice that she is a petite women and that her uterine size looks to be much
larger than you would expect for a 14wk pregnancy.
You decide to do her first ultrasound to confirm the GA and you see:
QUESTION:How would you describe this finding with in the uterine cavity?
Case 5• The ultrasound is a classic example of a SNOW STORM appearance with in the
QUESTION: What does this finding mean?
Case 5 Answer: This is an example of COMPLETE MOLAR PREGNANCY
MOLAR PREGNANCY:• A type of benign gestational trophoblastic pregnancy often called “hydatidiform mole”• 2 types: complete mole (no fetal parts) vs incomplete mole (partial fetal parts)
• Common presentation of Complete Molar Pregnancy:– Often have excessively higher than expected hCG levels for gestational age– abnormal painless vaginal bleeding– Uterine size larger than expected for gestational age
• Ultrasound findings with in uterine cavity:– Complete mole: pathognamonic “snow storm” appearance with absence of fetal heart beat or fetal
parts– Incomplete mole: presence of abnormal incomplete fetal parts with absence of fetal heart beat
BONUS QUESTION: What makes the patient at greater risk of having molar pregnancy?
Bonus Answer:• Possible risk factor: patient is Vietnamese• It has been thought that women from Southeast Asian descent are at higher risk of
having molar pregnancies.
The end…Of course there is plenty more utility of ultrasound in obstetrics than was
presented, but hopefully these basics will help you understand what’s going on with the cute little fetus!
References:• Beckman, C. Obstetrics and Gynecology. Baltimore: Lippincot Williams and Winkot, 2010. • Gjelsteen, A. et al. CT, MRI, PET, PET/CT and Ultrasound in the Evaluation of Obstetric and Gynecologic Patients. Surgical
Clinic North America 2008; 88: 361–390. • Khaled, M. et al. Imaging of the Placenta: A Multimodality Pictorial Review 1. RadioGraphics 2009; 29:1371–1391. Published
online . • Jauniaux, E.The role of ultrasound imaging in diagnosing and investigating early pregnancy failure. Ultrasound Obstet
Gynecol 2005; 25: 613–624.• ACOG. Ultrasonography in Pregnancy. American College of Obstetrics and Gynecology: Practice Bulletin. February 2009; No
101. Additional images obtained from:• http://www.emergencyultrasoundteaching.com/galleries/image_galleries/obgyn_images/index.php • http://onradiology.blogspot.com/2011/08/molar-pregnancysnow-white-appearance-on.htm• http://www.umm.edu/pregnancy/000247.htm• http://hcp.obgyn.net/ultrasound/content/article/1760982/1880108#• http://www.baby2see.com/development/ultrasound_sonogram/first_trimester_scans.htm• https://www.midcarolinaobgyn.com/nuchal.html