Abnormal Sac Position
Karen Y. Oh, MD
DIFFERENTIAL DIAGNOSIS
Common
Spontaneous Abortion
Tubal Ectopic
Less Common
Uterine Duplication
Interstitial Ectopic
Cervical Ectopic
C-Section Scar Ectopic
Rare but Important
Heterotopic Pregnancy
Abdominal Ectopic
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Confirm intrauterine pregnancy (IUP)
Exclude ectopic pregnancy
Beware of pseudosac: Fluid centrally located in endometrial canal
Should have typical signs of early IUP depending on gestational age
Intradecidual sac sign: Echogenic ring around sac within endometrium
Double decidual sac sign: Paired echogenic rings around early sac
Double bleb sign: Yolk sac and amniotic sac
Diamond ring sign: Yolk sac with associated early embryo
Clarify location of implantation site
Should be eccentric within endometrium
Sac should be within the body of the uterus above internal os
Myometrium should be completely surrounding sac
Helpful Clues for Common Diagnoses
Spontaneous Abortion
Use color Doppler to differentiate ectopic implantation from abortion in progress
Early IUP has increased surrounding flow; “ring of fire” appearance due to trophoblastic tissue
Spontaneous abortion is much less vascular
If embryo visualized, assess for heart beat
If heart rate detected, usually indicates an implanted pregnancy
Evaluate internal os
Will be open for abortion in progress
Closed in cervical ectopic
Correlate with serial hCG
Should be decreasing with spontaneous abortion
Tubal Ectopic
Most specific diagnostic clue: Adnexal gestational sac ± embryo
Most common presentation: No IUP, tubal/adnexal mass, echogenic free fluid in cul-de-sac
Decidual reaction in uterus
May have pseudosac
Heterogeneous tubal hematoma
Pulsed Doppler shows low resistance flow in tubal pregnancy
Ectopic often on same side as corpus luteum
Ultrasound negative in 5-10% of cases
Helpful Clues for Less Common Diagnoses
Uterine Duplication
May give the appearance of ectopic implantation
Implantation actually within one horn of uterine anomaly
Didelphys: 2 separate endometrial cavities
Bicornuate: 2 separate uterine horns with concave outer uterine contour
Septate: Variable length of septum separating cavities, normal outer uterine contour
Myometrium completely surrounds sac as implantation is normal
Interstitial Ectopic
Look for interstitial line sign
Echogenic line from endometrium to ectopic sac
Myometrium around sac should be at least 5 mm thick
Can grow to be larger than tubal ectopic as blood supply better
Within intramural portion of fallopian tube
May present as echogenic mass within cornua without sac
Mass is combination of trophoblastic tissue and hematoma
Cervical Ectopic
Prior instrumentation of uterus considered key risk factor
Assess for “hourglass” shape of uterus
Cervix distended but internal os is closed
Transabdominal ultrasound helpful to evaluate landmarks and shape
Eccentric implantation into wall of cervix
Distinguishes from spontaneous abortion in progress which is central
C-Section Scar Ectopic
Multiple prior C-sections may increase risk
Look for implantation near scar and thinned/absent anterior myometrium
Assess for other associated complications if presenting later in pregnancy
Placenta accreta, increta, percreta
Placenta previa
Placental abruption
Trophoblastic tissue in scar may invade into bladder
Helpful Clues for Rare Diagnoses
Heterotopic Pregnancy
Correlate with clinical history
< 1:30,000 in spontaneous pregnancies
Much more common if history of assisted reproduction
Damage to endometrium or fallopian tubes predisposes to ectopic implantation
IUP identified but adnexal mass seen
Beware of misdiagnosis due to “ring of fire” around corpus luteum
Intraovarian ectopics exceedingly rare
Tissue around corpus luteum can normally be hypervascular
Abdominal Ectopic
No IUP identified but sac or embryo/fetus seen outside the uterus
Lack of normal hypoechoic myometrial rim around pregnancy
Look for echogenic free fluid in abdomen and pelvis
If ruptured or early may only see hematoma
Other Essential Information
Always correlate with serum human chorionic gonadotropin (hCG)
If hCG > 2000 IU should see IUP
No IUP → missed or spontaneous abortion vs. ectopic pregnancy
If no IUP and patient stable, can follow serial hCG ± ultrasound
If hCG < 2000 IU may be too early to see IUP
Differential includes early IUP, spontaneous abortion and ectopic pregnancy
Look for signs of ruptured ectopic: Echogenic fluid, adnexal “mass”, pain, anemia
If patient stable, can follow with serial hCG and ultrasound
Normal hCG should double every 2-3 days in the first trimester
Ectopic pregnancies usually rise more slowly than normal IUP
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