4. 1st Trimester Obstetric/OB Ultrasound Protocol

Molar Pregnancy

A molar pregnancy is a tumor due to the uncontrolled proliferation of trophoblasts, which are cells that surround the blastocyst and later become the placenta. These “moles” are categorized into complete and partial.

Complete molesPartial moles
No fetal/embryonic tissueMay contain fetal structures, albeit abnormal ones
Abnormally elevated B-hCG levels: >100,000 mIU/mLNormal B-hCG levels


  • Vaginal bleeding
  • Exaggerated uterine size for gestational age
  • Hyperemesis
  • Pregnancy-induced hypertension

A molar pregnancy will appear on ultrasound as a “snowstorm” inside the uterus. The contents of the uterus are complex and heterogeneous and often contain many small cystic structures. In addition, there may be theca-lutein cysts in the ovaries which are thin-walled cystic structures (these are more common in 2nd-trimester complete moles).

Molar Pregnancy Snowstorm Appearance OB Obstetric Obstetrical Ultrasound
Molar pregnancy. Note the snowstorm appearance and the cystic structures.
Theca Lutein Cyst in Ovary OB Obstetric Obstetrical Ultrasound
Theca lutein cysts in the ovary.
4. 1st Trimester Obstetric/OB Ultrasound Protocol


It is important to use proper terminology when describing miscarriages:

Clinical ExamUltrasound Findings
Threatened MiscarriageAbnormal vaginal bleeding during a viable pregnancyNormal IUP
Inevitable MiscarriageVaginal bleeding with a dilated cervix while still retaining all products of conceptionNormal IUP or products of conception near cervix
Incomplete MiscarriageSome products of conception have been retainedRetained products of conception
Complete MiscarriageAll products of conception have been expelled and bleeding has stoppedEmpty, non-gravid uterus
Missed MiscarriageThe fetus is non-viable but no products of conception have been passedIUP with no cardiac activity or fetal movement
4. 1st Trimester Obstetric/OB Ultrasound Protocol

Corpus Luteal Cyst

A corpus luteal cyst develops due to the growth, instead of the normal regression, of the corpus luteum. These cysts will appear very similar to a tubal ectopic pregnancy.

How can you tell the difference between the two?

Tubal Ectopic PregnancyCorpus Luteal Cyst
May find yolk sac or fetal pole
Will move independent of the ovary in response to transducer manipulationWill move with the ovary in response to transducer manipulation
Tubal ring is more echogenicTubal ring is less echogenic
Tubal ring is thickerTubal ring is thinner
Cystic fluid is more likely to be “clumpy” or have some echoesCystic fluid is more likely to be clear and anechoic
May have ring of fire signMay have ring of fire sign
Corpus Luteum Cyst with Ring of Fire and Thin Tubal Ring Ultrasound
Corpus luteal cyst. Note how the fluid and ring are less echogenic than the ectopic pregnancies shown before. Note how the tubal ring is thinner than that of an ectopic.
4. 1st Trimester Obstetric/OB Ultrasound Protocol

Ectopic Pregnancies

An ectopic pregnancy refers to any pregnancy occurring outside of the uterus. They pose a substantial risk to the mother and thus must be diagnosed accurately. Ectopic pregnancies cannot be missed.

  • These patients commonly present with abdominal painpelvic pain, or vaginal bleeding.
  • Presentations that increase concern:
    • Peritoneal irritation
    • Cervical motion tenderness
    • Lack of fetal heart tones
  • Risk factors:
    • Previous tubal surgery
    • Prior ectopic pregnancy

The vast majority of ectopic pregnancies are found in the fallopian tubes, particularly in the ampullary region of the tube. They can be seen in many different locations, however. Because there are many places in which an ectopic pregnancy can be, the primary goal of ultrasound should be not to definitively rule in an ectopic pregnancy, but rather to rule in an intrauterine pregnancy; therefore making an ectopic pregnancy highly unlikely.

Ectopic Pregnancy Locations

Follow these steps to look for an ectopic pregnancy:

Step 1  Scan the uterus for a gestational sac with a yolk sac, fetal pole, or cardiac activity within the uterus. If detected then the patient has IUP and unlikely a concurrent ectopic (unless IVF therapy)

  • Step 2 – if no IUP detected then scan adnexa for direct signs of ectopic pregnancy.
    • The tubal ring sign  is a common ultrasound finding with an ectopic pregnancyThis is a thick hyperechoic ring around a tubal mass.
    • The ring of fire sign can be seen in ectopic pregnancy but can also be seen in corpus luteum cysts. The ring of fire sign describes the high-velocity flow seen on color Doppler imaging due to the high vascularization of the area surrounding the ectopic pregnancy.
    • Finding a gestational sac and fetal pole with cardiac activity outside of the uterus is diagnostic of an ectopic pregnancy.
Tubal Ring Sign Ectopic Pregnancy OB Obstetric Obstetrical Ultrasound
Thick echogenic ring around a tubal mass (left side). It is more echogenic than the corpus luteum (right side).
Ring of Fire Sign Ectopic Pregnancy OB Obstetric Obstetrical Ultrasound
Ring of fire sign in an ectopic pregnancy.

Step 3  Assess the Pouch of Douglas for free fluid.

Step 4 – Integrate ultrasound findings with B-hCG levels

  • Normal IUP
    • Fetus visible in the uterus
    • B-hCG level of 1000-2000 mIU/mL
  • Suspicious for Ectopic
    • Ectopic mass
    • Fluid in the cul de sac
    • Absent IUP
    • Abnormal serum B-hCG pattern*

*Normally, B-hCG levels rise at least 50% in a 48 hour period. In ectopic pregnancies, B-hCG levels commonly rise 50% or less in the same timeframe. Additionally, B-hCG levels plateau sooner than IUPs (IUPs usually plateau at around 10-12 weeks).

Ectopic Pregnancy Algorithm OB Obstetric Obstetrical Ultrasound
4. 1st Trimester Obstetric/OB Ultrasound Protocol

Cardiac Activity (Fetal Heart Rate Measurement)

Assessing cardiac activity is a crucial task in obstetric ultrasound. Fetal cardiac activity should be seen at a crown rump length (CRL) of 5-7mm.

M-mode is the preferred way to measure fetal cardiac activity over pulsed wave doppler because it subjects the fetus to lower ultrasound energy.

  • Step 1 – Locate the fetal heart and optimize the depth
  • Step 2 – Properly place the M-Mode cursor over the beating heart and activate M-Mode
  • Step 3 – Open the OB calculation package and place the vertical cursors at one heartbeat cycle (some machines measure at 2 heartbeat cycles)
  • Step 4– The machine should automatically calculate the fetal heart rate from your measurement.
Fetal heart rate measurement FHR OB Obstetric Obstetrical Ultrasound
Proper caliper placement and fetal heart rate measurement

Normal fetal heart rate ranges:

6-7 weeks100-120 bpm
8 weeks145-170 bpm
9+ weeks120-160 bpm
4. 1st Trimester Obstetric/OB Ultrasound Protocol

Pregnancy Dating in the 1st Trimester

Mean Sac Diameter (MSD)

Prior to the appearance of a fetal pole, the mean sac diameter can be used to estimate the gestational age.

  • Step 1 – Optimize depth to see gestational sac
  • Step 2 – Obtain a sagittal view of the gestational sac
  • Step 3 – Measure the height and length of the sac using the mean sac diameter calculation package.
  • Step 4 – Rotate the probe 90º to obtain a transverse view of the gestational sac.
  • Step 5 – Measure the width of the gestational sac.

Generally, the formula for calculating gestational age from mean sac diameter is:

MSD (in mm) + 30 = Gestational age (in days)

StructureFor transvaginal ultrasound, the structure should be seen by the time the MSD is:For transabdominal ultrasound, the structure should be seen by the time the MSD is:
Yolk Sac8 mm20 mm
Fetal Pole16 mm25 mm
MSD Mean Sac Diameter Measurement OB Obstetric Obstetrical Ultrasound
How to measure the mean sac diameter in 3 planes.

Crown-Rump Length (CRL)

Once a fetal pole is present, the crown-rump length should be used to estimate the gestational age as it is the most accurate method of dating the pregnancy. The crown-rump length is defined as the measurement between the top of the head and the bottom of the torso.

  • Step 1 – Optimize depth to clearly see the entire fetal pole or embryo.
  • Step 2 – Acquire a proper view of the fetus
  • Step 3 – Measure the crown-rump length using your machine’s calculation package.
Crown Rump Length CRL Measurement OB Obstetric Obstetrical Ultrasound
Proper fetal positioning and CRL measurement.

If your ultrasound does not have an OB package use this formula: CRL (in mm) + 42 = Gestational age (in days)

Tip: Make sure that the view of the fetus is in the mid-sagittal plane. Having an oblique section or an off-center longitudinal section of the fetus will lead to underestimation of the crown-rump length.

4. 1st Trimester Obstetric/OB Ultrasound Protocol

1st Trimester Obstetric/OB Ultrasound Protocol

Identifying an Intrauterine Pregnancy

To definitively diagnose an intrauterine pregnancy, either a yolk sac or a fetal pole must be seen within a gestational sac inside of the uterus.

Transvaginal UltrasoundTransabdominal Ultrasound
Gestational sac4-5 weeksNot reliable for gestational age under 6 weeks
Yolk sac5 weeks6-7 weeks
Fetal pole5-5.5 weeks7 weeks

Gestational Sac (4-5 weeks)

The gestational sac is a collection of fluid surrounding the embryo and yolk sac. It is the first structure to be seen in the development of an IUP. It will have these features:

  • Anechoic (dark), round structure with an echogenic (bright) border.
  • Typically, it is in the upper 1/3 of the uterine fundus.
  • You should expect to see the gestational sac at 4.5-5 weeks gestational age with transvaginal ultrasound.
  • Make sure the sac is actually in the uterus by tracking the vaginal stripe to the uterus.

Shortly after the appearance of the gestational sac, the decidual layers are commonly seen. This is called the Double Decidual Sign. They will appear as 2 echogenic rings surrounding the gestational sac. The outer ring is the decidua parietalis (lining the uterine cavity) and the inner ring the decidua capsularis (lining the gestational sac). The presence of a Double Decidual Sign is highly indicative of an early intrauterine pregnancy. However, a definitive diagnosis of IUP will require the presence of a yolk sac or fetal pole.

Double Decidual Sign
Double Decidual Sac Sign OB Obstetric Obstetrical Ultrasound
Double Decidual Sac Sign

Tip: Don’t diagnose a IUP until you see a yolk sac or fetal pole within a uterine gestational sac.

Pseudogestational Sac

A common pitfall is to falsely identify a gestational sac. Small collections of fluid can look very similar to gestational sacs and are appropriately called Pseudogestational sacsHow can we tell them apart? Pseudogestational sacs will usually have one or more of the following characteristics:

  • A pseudogestational sac is more irregularly shaped or pointy-edged than a round gestational sac.
  • The border surrounding the sac is not as echogenic as that of a true gestational sac.
  • The fluid within a pseudo-gestational sac is not completely anechoic, there are some echoes seen in the fluid.
  • The fluid of a pseudo-gestational sac will not be found in the decidua like a true gestational sac, but in the uterine cavity.
  • A pseudo-gestational sac will not have the contents of a maturing gestational sac such as the yolk sac and embryo.

Correctly distinguishing between a true and pseudo-gestational sac is important because intrauterine fluid collections reportedly occur in 9-20% of ectopic pregnancies. Additionally, misdiagnosing a pseudo-gestational sac for an early IUP can lead to improper treatment.

Note the irregular shape, lack of double decidual sign, presence of echoes in the fluid, and lack of embryonic contents in Pseudogestational Sac.

Pseudogestational sac
Pseudogestational Sac OB Obstetric Obstetrical Ultrasound
Sagittal view of a pseudogestational sac

Yolk Sac – (5 weeks)

Typically seen at around 5 weeks gestational age by transvaginal OB ultrasound, the yolk sac is an early source of nutrition for the developing embryo which usually isn’t yet visible. The yolk sac is a circular, echogenic ring with an anechoic center seen eccentrically (not in the center) in the gestational sac.

Yolk Sac with Gestational Sac OB Obstetric Obstetrical Ultrasound
Yolk sac present in the gestational sac.

Fetal Pole – (5.5-6 weeks)

The fetal pole, or developing embryo, should be seen at 5.5-6 weeks gestational age by transvaginal ultrasound. It grows directly adjacent to the yolk sac. With transvaginal ultrasound, the fetal pole should be seen when it is 2-4mm in length.

Yolk Sac with Fetal Pole Embryo OB Obstetric Obstetrical Ultrasound
Yolk Sac with Embryo Fetal Pole.