Irregular Gestational Sac

Reviewed by Dr. Temeka Zore OB/GYN, REICheckmark
Published: July 29, 2017

What Does Irregular Gestational Sac Mean?

An irregular gestational sac is the sac of fluid in which an embryo develops in very early pregnancy that has irregular or ill-defined borders in comparison to a normal gestational sac.

Gestational sacs can vary in shape to a certain degree, but an irregular gestational sac is usually a cause for concern as they are predictors of poor pregnancy outcomes.

Ultrasounds that show irregular gestational sacs are normally followed up by further ultrasounds or other imaging to confirm the diagnosis and determine the risk of miscarriage.

FertilitySmarts Explains Irregular Gestational Sac

In early pregnancy, three structures can be visualized on a transvaginal ultrasound including the fetal pole/embryo that becomes the fetus, the yolk sac that nourishes embryo, and the gestational sac that encloses the embryo and yolk sac.

These three structures can provide information regarding how the pregnancy is progressing.

A gestational sac is the first structure visible on ultrasound imaging as early as 4.5 weeks. When a gestational sac is visualized it can:

  • Be measured with a calculation called the mean sac diameter (MSD) to assess the development and estimate the gestational age.
  • Provide additional information about how the pregnancy is progressing based on the shape of the structure.

The appearance of a normal gestational sac:

  • Is round or elliptical
  • Is located in the upper body of the uterus called the fundus
  • Contains a smaller sac called the yolk sac
  • Contains the amniotic sac (which will eventually take up the entire sac)

The appearance of an irregular gestational sac:

  • Is generally larger than usual
  • Has a position low in the uterus
  • Has no embryo or yolk sac inside (with aMSD of 25 mm)
  • Has a distorted shape
  • Has thin walls

A gestational sac with one or more of these features may indicate that the pregnancy is not viable with the potential to end in spontaneous miscarriage.

Additional Concerning Factors

Additional factors associated with poor prognosis at this early stage of pregnancy include:

  • Discrepancy between the age and size (MSD) of the embryo.
  • Abnormal fetal heart rate. Around 6 weeks gestational age, a typical fetal heart rate is 100-120 beats per minute (BPM).

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