PoCUS in Early Pregnancy – a Resident Clinical Pearl (RCP)

Dr. Victoria Landry, R3

Integrated Family Medicine Emergency Medicine Program

Saint John, NB

Edited by Dr. Rawan AlRashed, PoCUS fellow

Copyedited by Dr. Mandy Peach

PoCUS use by the emergency physician for the diagnosis of uncomplicated intrauterine pregnancy have been proven to be affective in expiditing patient management and decreasing the length of stay in the emergency department. In a metanlaysis done by Stein et.al. emergency physiscain performed PoCUS was found to be 99.3% sensitive in ruling out ectopic pregnancy by detecting an Intauterine pregnancy (IUP). In this review, ultrasound findings in the first trimester will be highlighted.

Indication: Confirmed or suspected pregnancy with abdominal pain, vaginal bleeding, syncope, or hypotension⁽²⁾

Technique

Start with trans-abdominal ultrasound (TAUS) ^(1,2)

• Use abdominal probe (deep penetration, wide field view; use “obstetrics” or “gynecology” preset)
• Acoustic window is a full bladder (anechoic structure in the near field). Uterus is a homogenous structure beneath bladder
• Place abdominal probe midline longitudinally/sagitally immediately superior to symphysis pubis with probe marker toward patient’s head. Adjust depth so uterus is in middle of screen. Sweep left and right till uterus disappears in each view.
• Rotate probe 90° into transverse plane with marker toward patient’s right side. Sweep up and down till uterus disappears in each view.
• To improve image: Turn the gain down, sweep slowly

Then Consider the use of transvaginal ultrasound (TVUS) if available, and qualified to use  ⁽¹⁾

• Requires empty bladder, Patient in lithotomy position.
• Ultrasound gel on probe, latex condom over top (ensure no air bubbles), then sterile lubricant
• Reference mark toward ceiling (in sagittal orientation), insert 4-5cm into vagina, sweep left and right
• Turn probe 90° C to be in coronal plane and marker to the right of the patient – sweep anterior and posterior

General principles ⁽¹⁾

• follow the endometrial stripe (echogenic line within uterus) along its entire course (left to right in longitudinal view, cervix to fundus in transverse view), looking for evidence of a pregnancy
• You are trying to rule in an intrauterine pregnancy (IUP) (as opposed to rule out an ectopic) – assume all pregnancies are ectopic until proven otherwise⁽²⁾

Figure 1 – Longitudinal/sagittal view (TAUS): ⁽¹⁾

Figure 2 – Transverse view (TAUS): ⁽¹⁾

Discrimination zone (βHCG levels below which you cannot see an IUP)^{(2, 3)}

• TVUS – βHCG 1500-2000 mlU/ml
• TAUS – βHCG 5000-6000 mlU/ml
• If No definite IUP (NDIUP) above these levels, strongly consider ectopic!

Findings:

Inutrauterine pregnancy

• The “double ring sign” is the earliest sign of a definitive IUP. Diagnosing an intrauterine pregnancy (IUP) requires visualization of all 3 structures inside the uterus. ^(1,2)
• Decidual reaction – hyperechoic (white) line in uterus ⁽²⁾ represents endometrium thickening – begins around day 14 post-fertilization ⁽¹⁾
• Gestational sac – anechoic (black) round area within decidual reaction, contains amniotic fluid, seen at 4-5wks (TVUS), 6wks (TAUS) ⁽²⁾
• Yolk sac +/- fetal pole within the gestational sac⁽²⁾
  • Yolk sac: circular echogenic layer, looks like a cheerio, visible when gestational sac is 10mm by TVUS (~5-6wks GA), 20mm by TAUS (~6-7wks GA) ⁽¹⁾
  • Fetal pole: echogenic structure; develops around the same time as yolk sac but visualized on US ~1wk later⁽¹⁾

Figure 3 – Double ring sign⁽¹⁾

Figure 4 – Double ring sign⁽⁴⁾

Figure 5 – Fetal pole⁽¹⁾

Measurements

Mean sac diameter

• Obtain sagittal view of gestational sac, measure height and length of sac using mean sac diameter calculation package, rotate probe 90º to obtain transverse view of gestational sac, measure width of sac
• MSD (mm) + 30 = Gestational age (days)

Crown-rump length (CRL) = Top of skull to base of pelvis⁽¹⁾

• >5mm without visible fetal heart = unlikely to proceed to viability
• CRL (mm) + 42 = gestational age (days)
• The most accurate method of dating the pregnancy⁽³⁾

Fetal cardiac activity = proof of live IUP⁽¹⁾

• detectable ~6wks on TVUS (fetal pole is >5mm), 7-8wks on TAUS (fetal pole is >10mm) ⁽¹⁾
• Normal IUP with fetal cardiac activity is reassuring!
  • absence of cardiac activity will likely result in miscarriage, presence of cardiac activity reduces risk of miscarriage (HR >100 consistent with good fetal outcome)
• Technique⁽³⁾
  • Locate fetal pole, optimize depth, turn on M-mode (never doppler as it subjects fetus to high US energy and may be harmful)^(1,2), place caliper over beating heart, measure and calculate heart rate
  • Note: must be within gestational sac, well away from uterine wall (don’t confuse with highly vascular decidual reaction)⁽¹⁾
  • Normal FHR Ranges
    • 6-7wks: 100-120bpm
    • 8wks: 145-170bpm
    • 9+wks: 120-160bpm

Other findings and descriptions

No definitive intrauterine pregnancy (NDIUP) ⁽²⁾

• if any single criteria of IUP is missing

DDx for NDIUP⁽²⁾:

• Early normal pregnancy (βHCG below discrimination zone)
• Threatened/spontaneous abortion
• Anembryonic pregnancy (blighted ovum)
• Molar pregnancy
• Ectopic pregnancy

Threatened abortion: abnormal bleeding during pregnancy; normal IUP on US⁽³⁾

Inevitable abortion: vaginal bleeding with open os^; normal IUP or product of conception (POC) near cervix on US⁽³⁾

Incomplete abortion: open os with retained POC; US shows anything from debris to embryo; abnormal uterine contents confirms dx⁽¹⁾

Complete abortion: empty uterus + positive βHCG +/- closed os; same findings as for ectopic therefore requires formal US + serial βHCG⁽¹⁾

Ectopic pregnancy ⁽³⁾

• NDIUP (no definitive intrauterine pregnancy) above βHCG in discriminatory zone
• Scan adnexa for signs of ectopic
  • Tubal ring sign (thick hyperechoic ring around a tubal mass)
  • Ring of fire sign (also seen in corpus luteum cysts; high velocity flow seen on color doppler around the
  • gestational sac + fetal pole with cardiac activity outside the uterus is diagnostic of an ectopic
• assess pouch of douglas for free fluid
• suspicious for ectopic: ectopic mass, fluid in cul de sac, absent IUP, abnormal βHCG pattern (normally rises at least 50% in 48hr period)

Corpus luteal cyst^(2,3)

• develops due to growth, instead of normal regression, of corpus luteum
• appears very similar to ectopic, but will move with the ovary in response to transducer manipulation instead of independent, tubal ring is thinner and less echogenic, cystic fluid is more clear and anechoic (rather than “clumpy” with echoes)
• ovarian cyst characteristics: outside the uterus, circular, well circumscribed, do not taper to solid organs

Blighted ovum (anembryonic pregnancy)^(1,2)

• abnormally large gestational sac with no embryonic contents
  • gestational sac >20mm without yolk sac visible à suspect blighted ovum
  • >25mm without yolk sac visible à blighted ovum virtually certain (Eliminates diagnosis of ectopic)
• Positive βHCG (higher than expected for GA)
• Confirm with formal US

Molar pregnancy ^(1,3)

• Tumor due to uncontrolled proliferation of trophoblasts (cells that surround blastocyst and later become the placenta)
• Complete mole: no fetal/embryonic tissue; abnormally elevated βHCG >100,000 mIU/ml
• Partial mole: may contain (abnormal) fetal structures
• Presentation: hyperemesis, larger uterus than expected, vaginal bleeding, anemia, signs of hyperthyroidism, pregnancy-induced hypertension
• US: appears as a “snowstorm” or “cluster of grapes” in uterus – fairly homogenous mass full of small, fluid-filled (black) holes; no detectable fetal cardiac activity
• Needs gyne referral for surgical evacuation⁽²⁾

Pitfalls

• Pseudogestational sac ^{(1, 3)}
  • contains no yolk sac, usually more irregularly shaped or pointy-edged than a true gestational sac, border is not as echogenic, and fluid may contain some echoes
  • Intrauterine fluid collections occur in 9-20% of ectopic pregnancies
  • Unless all 3 criteria met for double ring sign, pt requires formal US
• Extrauterine pregnancy⁽¹⁾
  • Recognize uterine tissue and always confirm bladder-uterus juxtaposition⁽²⁾
• Interstitial and cornual ectopic pregnancies⁽¹⁾
  • Rare but dangerous – tend to rupture later therefore produce more rapid hemorrhage than other ectopics
  • Myometrium around interstitial and cornual pregnancies is thin and uneven⁽²⁾
  • Measure the “myometrial mantle” (the thinnest part of myometrium around the gestational sac) – should be >5-7mm thick (thinner is concerning for cornual or interstitial ectopic pregnancy) ⁽²⁾
• Multiple pregnancies⁽²⁾
  • In multiple gestation, each fetus needs to meet the criteria for IUP
  • Heterotopic pregnancies = combined IUP and ectopic pregnancy
    • Risk is 1:30,000 in general population
    • Risk increases to 1:100 with fertility treatment (e.g. IVF)

Figure 7 – Extrauterine pregnancy⁽¹⁾

Figure 8 – Normal myometrial mantle⁽¹⁾

Figure 9 – Cornual ectopic pregnancy⁽¹⁾

Key points⁽¹⁾

• False positive IUP can have devastating consequences
• Any positive βHCG + no definitive IUP = presumed ectopic
  • Pt stable + no free fluid à formal US + quantitative βHCG
    • If no ectopic mass, repeat formal US and βHCG in 48hrs with consideration of patient risk of ectopic pregnancy
    • Follow up with OB to be arranged
  • Always consider other diffrerntail diagnosis for patient presentation before discharging them home.

Figure 10 – Clinical application⁽²⁾

References:

1. Socransky, S., & Wiss, R. (2016). Obstetrical EDE. In Essentials of point-of-care ultrasound: The EDE book (pp. 61-90). The EDE 2 Course.
2. Long, N. (2020, March 02). VanPOCUS: 1st Trimester Obstetrics • LITFL • Ultrasound Library. Retrieved October 15, 2020, from https://litfl.com/vanpocus-1st-trimester-obstetrics/
3. Dinh, V. (n.d.). Obstetric/OB Ultrasound Made Easy: Step-By-Step Guide. Retrieved October 15, 2020, from https://www.pocus101.com/obstetric-ob-ultrasound-made-easy-step-by-step-guide/
4. Flores, B., Smith, T., & Joseph, J. (n.d.). OB/Gyn. Retrieved October 15, 2020, from https://www.thepocusatlas.com/obgyn-1