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.

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