Gestational Sac

Reviewed by Dr. Temeka Zore OB/GYN, REICheckmark | Last updated: April 28, 2021

What Does Gestational Sac Mean?

A gestational sac is a fluid-filled structure that encloses a developing embryo in the very early stages of pregnancy. It is the first visual evidence of pregnancy before the embryo is even visible.

It can be identified via transvaginal ultrasound at approximately 4.5-5 weeks gestation when it is approximately a 2-3mm rounded collection of fluid. A gestational sac may indicate whether or not a typical pregnancy is developing by providing information on:

  • If an early pregnancy is developing properly.
  • If the location of the pregnancy is in the uterus or it is an ectopic pregnancy, which is a medical emergency.
  • The number of fetuses that are developing.
  • An estimate of the gestational age, or how far along the pregnancy is.

FertilitySmarts Explains Gestational Sac

Once you've had a positive home pregnancy test and a blood test that shows proper levels of the pregnancy hormone human chorionic gonadotropin (hCG), the next step in verifying a clinical pregnancy is an ultrasound.

Why is a gestational sac important in early pregnancy?

At an ultrasound appointment to verify pregnancy, you might hear your provider talk about the visibility or appearance of a gestational sac.

Because the first visible structure in early pregnancy is the gestational sac, the appearance (or lack of appearance) can provide insight into the health of a pregnancy and help to estimate gestational age, which is how far along the pregnancy is.

Depending on the timing of the scan, a typically developing early pregnancy will show 3 structures including:

  • Gestational sac. Surrounds and contains the fetal pole and yolk sac and is visible at approximately 4.5- 5 weeks gestational age.
  • Yolk sac. This structure provides nutrients to the developing embryo and is visible at around 5.5 weeks gestational age.
  • Fetal pole. This is the developing embryo and is visible at around 6 weeks gestational age.

The Appearance of a Normal Gestational Sac

The gestational sac is like a bubble that encloses the embryo and is filled with amniotic fluid. Features of a normal gestational sac as they appear on ultrasound include:

  • A dark sphere surrounded by a thin, white ring.
  • It is a round or elliptical shape.
  • It is located within the upper part of the uterus called the fundus.
  • Intradecidual sign is seen before 5 weeks.
  • Double decidual sac sign, the appearance of two small “bubbles,” is seen just after 5 weeks.
  • The yolk sac is the first structure to appear around 5-6 weeks and is confirmation that the pregnancy is growing in the uterus, where it is supposed to be.

Gestational Sac Image

Measuring a Gestational Sac

Starting as early as 4.5 weeks, gestational age can be estimated based on the diameter of the gestational sac. This measurement is called the mean sac diameter (MSD). This measurement is taken in millimeters (mm) and can be used from 5/6 to 11 weeks, but is only used until the fetal pole/embryo can be seen, at approximately 6 weeks. While the growth rate varies from person to person, it is around 1.13 mm per day.

  • A gestational sac at 4.5 gestational weeks should measure approximately 2-3 mm.
  • A gestational sac at 5 gestational weeks should measure around 5-6 mm
  • A gestational sac at 6 gestational weeks should measure around 14 mm
  • A gestational sac at 7 gestational weeks should be around 27 mm, but the embryo is usually visible at this point, and it becomes the structure that is measured with crown-rump length (CRL).
Gestational Age in Weeks
Mean Sac Diameter (MSD) in mm
4.5
2-3
5
5-6
6
14

Gestational Sac Concerns in Early Pregnancy

There are several situations regarding a gestational sac that may indicate concerns regarding an early pregnancy:

Empty Gestational Sac

An empty sac means that there is no yolk sac or embryo at a point where there should be. An embryo should be seen when the MSD measures 25 mm or greater. This occurs when an embryo implants but fails to survive.

In this case, the body responds to early pregnancy triggers, and the gestational sac is developed; however, the embryo does not continue to grow inside.

This early type of miscarriage is called an anembryonic pregnancy (or a blighted ovum) and most likely occurs due to chromosome abnormalities of the embryo.

Another possibility is that the gestational age is incorrect, and the pregnancy is not as far along as originally thought.

Small Gestational Sac

A smaller than expected sac, defined as an MSD of approximately 4.5 mm (when the average is approximately 8.2 mm) at around 6 - 7 weeks gestational age (but not before), indicates that the pregnancy is not progressing normally and is predictive of a miscarriage. A small gestational sac at 4-5 weeks gestation is not predictive of a miscarriage and has the potential to become regular size.

Yolk Sac Fails to Develop

Starting between 5 and 6 weeks, a smaller sac called the yolk sac should appear inside the gestational sac. The yolk sac is responsible for the nourishment of the embryo before the placenta forms.

The presence or absence of the yolk sac (and its shape) is a predictor of poor pregnancy outcomes.

No Gestational Sac is Visible

Following a positive pregnancy test, a lack of gestational sac may indicate an ectopic pregnancy, where implantation has occurred outside of the uterus and is not viable. This is a medical emergency.

A Note On Early Loss

The loss of a pregnancy can be devastating no matter how early on. It's OK to feel however you feel—a loss is still a loss.

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