US is not a requirement for medication abortion or uterine aspiration. US can be used when clinical dating is uncertain, to determine pregnancy location, viability, and / or provide procedural support.

Whether to use transabdominal or transvaginal US depends on patient preference, equipment availability, gestational age, and sonographer skill. Transabdominal US may be used to confirm intrauterine pregnancy (IUP) and assess gestational age; it is often preferred by patients, although transvaginal US is often helpful with earlier pregnancies (Fu 2018).

Transabdominal Probe Transvaginal Probe
  • External probe
  • Easy to prepare and clean probe
  • Better view with full bladder
  • Difficult to detect pregnancy <6 weeks LMP
  • Good for later pregnancy scanning
  • Body habitus and bladder may affect image quality
  • Internal probe
  • Need to prepare and clean probe properly
  • Better view with empty bladder
  • May detect pregnancy as early as 4.5-5 weeks LMP
  • Can see early pregnancy landmarks
  • With probe close to pregnancy, body habitus does not affect images
  • Improved ability to perform systematic scan

A limited first trimester US exam must include: (NAF CPGs 2022)

  • Uterine scan in both longitudinal and transverse planes to confirm IUP
  • Evaluation of pregnancy number (singleton or multiple gestation)
  • Measurements to document pregnancy dating
  • Evaluation of pregnancy landmarks, such as yolk sac, embryonic pole, and the presence or absence of fetal/embryonic cardiac activity

When Performing US

  • Ask if the patient wants to view the image, and be informed of multiple gestations or other pregnancy findings.
  • Inform the patient that US is being used only to confirm the location and dating of the pregnancy and is not a diagnostic US.
  • Consider starting your scan with transabdominal US, and switching to vaginal US only if you are unable to effectively visualize the pregnancy.
  • For vaginal US, use a non-latex probe cover, with US gel inside and lubricating jelly outside. Ask if the patient would prefer to self-insert the probe.
  • Systematically scan in the longitudinal and transverse planes.
  • Use clear and simple language to discuss the US findings with the patient.


TVUS planes (Image ARMS 2007)

  • Longitudinal view is used to confirm pregnancy is intrauterine. For longitudinal view, marking is up (12 o’clock) and uterus is scanned side to side, from ovary to ovary. This view should show uterine fundus connected to cervix with pregnancy inside the uterus.

ultrasound longitudinal view(Images AIUM Image Library: Obstetrics 2018)

  • Transverse view is used for dating and pregnancy landmarks, to evaluate for multiple gestations, and to obtain a full 3D image of the uterus. For transverse view, the probe is turned 90 degrees to the patient’s right (counterclockwise or notch turned to 9 o’clock) and uterus is scanned anterior to posterior, from fundus to cervix.

Clinicians should understand the sonographic pregnancy features that should be visible based on the patient’s last menstrual period.

Pregnancy Landmarks by Weeks LMP
Gestational Sac 4.5 – 5 weeks LMP
Yolk Sac 5.5 weeks LMP
Embryonic Pole 6 – 6.5 weeks LMP
Cardiac Activity 6 – 6.5 weeks LMP

     *Above landmarks are better characterized using transvaginal US


The Gestational Sac

  • Gestational Sac (GS) is the first evidence of pregnancy on US, as early as 4.5 weeks LMP; should always be seen by 5 weeks 5 days LMP by TVUS (Barnhart 2012).
  • Although location of a pregnancy cannot definitely be diagnosed as intrauterine until a yolk sac or embryo is seen (Richardson 2015), a gestational sac still has a high likelihood of being an IUP even in the absence of certain sonographic features if there is no adnexal mass (Phillips 2020, Benson 2013).
  • A true gestational sac should be located in the mid to upper portion of uterus, be eccentric (not midline) to endometrial canal, be round or oval in shape, and have a double decidual (or double ring) sign, as demonstrated by the FEEDS mnemonic.
  • Meeting these criteria does not completely exclude ectopic pregnancy (Fjerstad 2004)
    • F – Fundal (in mid to upper uterus)
    • E – Elliptical or round shape in 2 views
    • E – Eccentric to the endometrial stripe
    • D – Decidual reaction (surrounded by a thickened choriodecidual reaction; appears like fluffy white cloud or ring surrounding sac)
    • S – Size > 4 mm (soft criteria)

Gestational Sac vs. Pseudosac

Gestational Sac

Compared to the GS, the pseudosac is more irregular, central, smaller, and without a decidual reaction, and can be seen with an ectopic pregnancy. Note the “beak-shaped” appearance of the pseudosac in the image below. This can look similar to an early GS, although only may meet the F (fundal) criteria of FEEDS. Pseudosac may also appear as a mid-uterine small fluid collection.

Image: 2020



(May be associated with ectopic)

Image: Fjerstad M, et al. CAPS, 2004 Image: Dr Matt A. Morgan, Radiopaedia.org, rID: 34217

The Yolk Sac

The Yolk Sac (YS) is the first single US finding that confirms an IUP. The YS is a round echoic ring with anechoic (dark) center seen within GS. It appears typically at 5 ½ weeks when the MSD is 5-10 mm. The YS should not be included when taking a measurement of the embryo. The size of the YS is not diagnostic.

The Embryo and Cardiac Activity

The embryo follows predictable development and therefore size can be used to date a pregnancy. The embryo appears at approximately 6 weeks and grows 1 mm per day until 12-14 weeks. See pregnancy dating below using embryonic and fetal measurement. Cardiac activity appears around 6 ½ weeks.


Image from AIUM 2018

Establishing Pregnancy Viability

The following data on viability evaluated patients who desired to continue their pregnancies (Doubilet 2013). If the patient does not desire to continue the pregnancy, there is no reason to delay an abortion to wait for confirmation of viability. If the patient desires to continue the pregnancy, and findings are suggestive of early pregnancy loss (see table below), repeat US in 7-10 days.


Guidelines for TVUS Diagnosis of Early Pregnancy Loss in a Patient with an IUP of Uncertain Viability

(Adapted from Doubilet 2013, ACOG 2018)

  • CRL 7+mm and no cardiac activity
  • MSD 25+mm and no embryo
  • Absence of embryo with cardiac activity:
    • 2+ weeks after a scan that showed a gestational sac without a yolk sac
    • 11+ days after a scan that showed a gestational sac with a yolk sac
  • CRL 5-7mm and no cardiac activity
  • MSD 16-24mm and no embryo
  • MSD 13 mm or more and no YS
  • Absence of embryo with cardiac activity:
    • 7-13 days after a scan that showed a gestational sac without a yolk sac
    • 7-10 days after a scan that showed a gestational sac with a yolk sac
  • Absence of embryo 6+ weeks after LMP
  • Empty amnion (amnion seen adjacent to yolk sac with no visible embryo).
  • Enlarged yolk sac (>7mm)
    • Small gestational sac in relation to the size of the embryo (<5mm difference between MSD and CRL)


Gestational Sac Measurement and Calculation of Gestational Age:  
Mean sac diameter (MSD) is used for pregnancy dating before embryo is visible.

Measure 3 dimensions in 2 planes (from inside double ring to inside double ring):

  • Longitudinal Plane: Length (L) & Height (H)
  • Transverse Plane: Width (W)

Length and height measured in longitudinal view.

Width measured in transverse view.
Calculate the Mean Sac Diameter (MSD):

  • MSD = (L + W + H)/3
Calculate the Gestational Age (GA):

  • GA (in days) = MSD (in mm) + 30
Crown Rump Length (CRL) Measurement and Calculation of Gestational Age:
  • CRL = fetal pole (in mm)
  • Long axis not including limbs or YS

Calculate: GA (days) = CRL (mm) + 42

Image: AIUM 2018

Biparietal Diameter (BPD) Measurement
  • > 14 weeks, using the fetal BPD is preferred to CRL.
  • Inside to outside of skull circumference
  • At the level of the thalamus
  • No nuchal or eye structures

Image: AIUM 2018



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