What Is a Gestational Sac? Meaning, Shape, Size & Ultrasound Guide

When you discover you are pregnant, it may be exciting, but so too can come many questions when you discover you are pregnant early via an ultrasound scan. Gestational sac is one of the first terms you'll hear about when you take an early pregnancy scan.

Therefore, what is gestational sac, and what does it look like when it is seen on an ultrasound? The gestational sac is an early pregnancy structure that develops inside the uterus and envelops the early pregnancy. It is typically one of the first to be detected on an early pregnancy ultrasound.

The size, shape, location and development of the sac can yield useful information regarding early pregnancy. Occasionally, an ultrasound will report an irregular gestation sac and this can cause concern. A few times, though, it's not the case that an irregular appearance means the pregnancy won't develop normally.

What Is a Gestational Sac?

In early pregnancy, a fluid-filled sac forms in the uterus, known as a gestational sac. It develops after the implantation (after the first few weeks of pregnancy).

It can be seen during an ultrasound as a small, round or oval object inside of the uterus. It is one of the earliest signs of the pregnancy seen on the ultrasound.

Doctors consider the location, appearance and development of the gestational sac when they evaluate an early pregnancy, so it is important to know what it is.

A transvaginal ultrasound will often be able to determine early pregnancy structures before an abdominal ultrasound.

What Does Sac in Pregnancy Mean?

The term sac in pregnancy usually is used to describe the gestational sac during an ultrasound exam.

Once in place, a pregnancy grows within the uterus, and the gestation sac grows around the pregnancy. Other structures (yolk sac, fetal pole) can be seen within the gestational sac as pregnancy advances.

An early finding that is important is seeing a gestational sac in the uterus. But just because there's a sac present doesn't mean that a pregnancy is healthy. Its size, form and location in connection with other ultrasound findings are taken into account by doctors.

When Can a Gestational Sac Be Seen?

The gestational sac usually can be seen on a transvaginal ultrasound around 4.5 to 5 weeks of pregnancy, but this can vary from one pregnancy to another.

The time it is visible may be influenced by:

Date of ovulation
Date of implantation
The accuracy of the last menstrual period (LMP).
Pregnancy dating
Type of ultrasound
Individual pregnancy development

In instances when ultrasound is done very early, the gestational sac can be seen without a yolk sac or embryo. This does not necessarily indicate a problem. A subsequent scan may be advised at a later time.

What Does Single Intrauterine Gestational Sac Mean?

When the ultrasound report indicates single intrauterine gestational sac, it usually refers to one uterine sac being detected.

Single" refers to a sac observed while "intrauterine" refers to a sac within the uterus.

Thus, single intrauterine gestational sac means the presence of an early pregnancy sac in the uterine cavity.

This is an early positive development as the sac is within the uterus. Additional ultrasound usually is required to determine the progression of the pregnancy, though.

Doctors may search for:

Yolk sac
Fetal pole
Embryo
Cardiac activity
Appropriate growth

Irregular Gestational Sac: When the gestational sac is not as usual in shape or appearance when viewed on an ultrasound.

What Does an Irregular Gestational Sac Mean?

Normally developed irregular gestational sac have a well-defined shape and are often round or oval. However, sometimes, the sac may be elongated, distorted or irregular.

The irregular appearance may be due to:

Very early pregnancy
Uncertain pregnancy dating
The variation in early pregnancy development
Changes in the uterus.
Risks of abnormal pregnancy development

An irregular gestational sac does not necessarily mean a miscarriage. Other ultrasound findings and the clinical history are taken into account by doctors when deciding if any further evaluation is warranted.

Can Irregular Gestational Sac Become Regular?

Many ask, can irregular gestational sac become regular?

This will depend on the cause of the irregular sac and the stage of the pregnancy.

If the ultrasound is taken very early, the shape of the gestational sac might not be clearly defined. A follow-up scan may be easier to do as pregnancy progresses to determine the pregnancy's appearance.

At times, however, the irregular gestational sac may indicate abnormal development of pregnancies. So, it's not possible to predict its results based on a single ultrasound scan.

Another scan might be advised to assess:

Increase in the gestational sac
Yolk sac development
Fetal pole
Embryo development
Cardiac activity
Overall pregnancy progression

If irregular is seen at one early scan, a decision must not be made regarding the sac. Repeated imaging will give a more detailed picture.

What Can Be Seen Inside the Gestational Sac?

Further structures may be seen as the pregnancy goes on.

Yolk Sac

Early structure within the gestational sac is the yolk sac. Aids in the initial development of pregnancy.

The yolk sac can be seen from about week 5, but there is variation.

Fetal Pole

Early ultrasound appearance of an embryo is the fetal pole. May appear at about the 5th or 6th week, depending on dating of pregnancy and timing of the ultrasound.

Embryo and Cardiac Activity

As the pregnancy progresses, the embryo is better seen. During a first ultrasound, cardiac activity may also be able to be detected.

The absence of an embryo or cardiac activity in a very early scan is not necessarily a problem.

What Does a Normal Gestational Sac Look Like?

The normal gestational sac is typically:

In the uterus, or womb
Ovoid, rounded or oval-shaped.
Well-defined
Appropriate in size for the pregnancy stage
As development goes, related to expected pregnancy structures

The mean sac diameter (MSD) can be measured by doctors. This measurement should be taken in conjunction with the other ultrasound results.

What Does an Empty Gestational Sac Mean?

In some cases, an ultrasound may detect a gestational sac but fail to detect a yolk sac or an embryo.

This may occur in early pregnancy or if there is a discrepancy in the timing of pregnancy dates. Occasionally, however, the sac may be empty, and the pregnancy is referred to as anembryonic pregnancy, otherwise known as a blighted ovum.

If your baby is not seen on the first scan, doctors don't automatically conclude that the pregnancy is abnormal. Additional ultrasound may be required, and ultrasound criteria may be required.

Human chorionic gonadotropin (hCG) is a hormone that is manufactured during pregnancy. hCG can be measured in the blood and can be used to help evaluate early pregnancy.

But hCG is different for different people. The hCG test alone is not enough to decide if a pregnancy is progressing normally.

Doctors may consider:

Levels and trends of hCG.
Ultrasound findings
Pregnancy dates
Symptoms
Medical history

What If the Gestational Sac Is Not Seen?

When a pregnancy test is positive and a gestational sac is not seen on an ultrasound, there could be several reasons, such as:

Very early pregnancy
Incorrect pregnancy dating
Pregnancy that is not developing normally
Can develop during a period of pregnancy when the location of the embryo is unknown.
Ectopic pregnancy

Ectopic pregnancy is a pregnancy that starts outside the uterus, typically in one of the fallopian tubes.

When a positive pregnancy test is accompanied by severe abdominal or pelvic pain, heavy bleeding, dizziness or fainting, it is important to get medical attention immediately.

When Should an Ultrasound Be Repeated?

A follow-up ultrasound might be recommended if:

Pregnancy is quite new.
The gestational sac is the only feature seen.
The yolk sac/fetal pole is not seen.
The gestational sac looks abnormal
The dates of pregnancy are not certain.
The vagina is bleeding or there are pains in the abdomen.
A preliminary scan will not be able to confirm pregnancy development.

Repeated ultrasound provides information on growth of the sac and development of the expected structures of pregnancy.

Frequently Asked Questions

What is gestational sac?

A gestational sac is a very early pregnancy formation which grows within the uterus and encloses the developing pregnancy. It is one of the first structures seen in an early ultrasound.

What is a sac in pregnancy?

In pregnancy, a sac is typically used to describe the gestational sac, which is the fluid-filled bubble that envelops the early pregnancy.

What does single intrauterine gestational sac mean?

Single intrauterine gestational sac means when one gestational sac has been detected within the uterus. Additional scans may be needed to check on the progress of pregnancy.

Can an irregular gestational sac become regular?

If the gestational sac is irregular, it might look more regular at a later scan, especially if the gestational sac is very early. Follow-up with ultrasound is necessary to follow the pregnancy.

Is an irregular gestational sac always a sign of miscarriage?

No. If a gestational sac is not round, it does not necessarily indicate that a miscarriage will happen. There are several ultrasound characteristics that help doctors reach a diagnosis.

When can the yolk sac be seen?

Around the fifth week of pregnancy the yolk sac is visible, but this can differ.

When does the fetal pole appear?

Around 5 to 6 weeks of pregnancy (depending on pregnancy dating and timing of the ultrasound), the fetal pole may be detected.

Consult a Maternity Specialist in Kamothe

There is some concern about the gestational sac. If something is abnormal or you are having concerns about an empty gestational sac, or concerns are raised from the scan results, it is important to discuss the scan with an experienced obstetrician to understand what your scan is showing and what to expect in the future.

In Kamothe, Matoshree Multispeciality Hospital is one of the Best Maternity Hospital in Navi Mumbai where mothers at various stages of pregnancy can get maternity care and gynecology care.

Make an appointment with Matoshree Hospital today to receive quality pregnancy and maternity treatment in Kamothe.

Conclusion

One of the first things that doctors can see on an ultrasound is the gestational sac. Understanding what is gestational sac, what a sac in pregnancy represents and what a single intrauterine gestational sac means can help you better understand your early pregnancy scan.

An irregular gestational sac may be worrying, but this does not necessarily mean a pregnancy will fail. For those who are wondering, "can irregular gestational sac become regular?" the most effective method to determine this is through follow-up ultrasound and professional medical assessment.

If you have an irregular sac, an empty sac, a yolk sac or a fetal pole mentioned in your ultrasound report, discuss this with your obstetrician or fertility specialist.