When Does the Gestational Sac Become Visible on an Ultrasound?

Doctor using ultrasound on pregnant woman
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The presence of a gestational sac on an ultrasound, especially when correlated with HCG levels, can be very helpful in detecting an ectopic or heterotopic pregnancy. It is also important in determining if you may have had a very early miscarriage.

When is a Gestational Sac Visible on Ultrasound?

It is important to note that, in early pregnancy, a transvaginal ultrasound is much more accurate than an abdominal ultrasound, so we will talk about findings on a transvaginal ultrasound alone.

In viable pregnancies, a transvaginal ultrasound should be able to detect the gestational sac by 5 weeks gestational age. The gestational sac is usually the first sign of pregnancy on ultrasound and may be seen as early as 3 weeks. At this time the average diameter of the sac is 2 to 3 millimeters. At around 5.5 weeks the yolk sac often becomes visible inside the gestational sac.

When correlated with hCG levels, a gestational sac should be seen on ultrasound when the hCG level when it has reached about 1500 to 2000.

What Does it Mean if the Gestational Sac is not Seen at 5 Weeks?

If it has been five weeks since your last menstrual period but your hCG level has not been measured, there is a possibility that seeing no gestational sac simply means that you ovulated late and your pregnancy has not yet reached 5 weeks gestational age. Your physician may order a follow-up ultrasound in a few days or a week.

If your hCG level is higher than 1500 to 2000 and the gestational sac is not visible, your doctor may diagnose an ectopic pregnancy.

If you have not had hCG measurements or if your hCG level is lower, your physician will probably order a follow-up ultrasound or continued monitoring of your hCG level.

It is also possible that the explanation is a very early miscarriage, aka chemical pregnancy, meaning that your pregnancy stopped developing before the gestational sac became large enough to see on the ultrasound.

Why Is Ultrasound Done in Early Pregnancy?

The main reason that an ultrasound is done in early pregnancy is to detect an intrauterine pregnancy and rule out an ectopic pregnancy. If an ectopic pregnancy is detected, ultrasound is useful when figuring out how to manage it. The goal of early ultrasound is not necessary to determine viability and fetal age of an intrauterine pregnancy—however, both viability and fetal age are determined if intrauterine pregnancy is detected.

What is an Ectopic Pregnancy?

An ectopic pregnancy is a pregnancy in which the fertilized egg doesn't implant in the wall of the uterus as it should. The fertilized egg needs to implant in the uterus to develop properly.

In an ectopic pregnancy, the fertilized egg sets up home in the fallopian tube, the tube which carries the egg from the ovary to the uterus. When the fertilized egg is stuck in the fallopian tube, it can do great damage to the tube, and if left untreated, can rupture or burst the fallopian tube causing internal bleeding and shock.

This is why it's very important that ectopic pregnancy is ruled out.

What is a Heterotopic Pregnancy?

In the vast majority of cases, detection of intrauterine pregnancy means that no ectopic pregnancy is present. However, in 1 of 3000 pregnancies, an intrauterine pregnancy, and an ectopic pregnancy may coexist—in other words, a twin pregnancy with one fetus implanted in the uterus and the other in the fallopian tubes.This phenomenon is called a heterotopic pregnancy.

In recent years, the proliferation of in vitro fertilization and administration of fertility drugs has resulted in more cases of heterotopic pregnancies.

One study examining outcomes of 725 in vitro fertilization pregnancies found that about 4 percent of patients had ectopic pregnancies and 2 patients had a heterotopic pregnancy.

Ultimately, women who receive fertility treatments and in vitro fertilization should be carefully screened for heterotopic pregnancy. In women who have not had fertility treatment, the presence of an intrauterine pregnancy usually means that an ectopic pregnancy is unlikely.

A Word From Verywell

At the present time, many ectopic pregnancies are diagnosed in the emergency room using bedside ultrasound. Because an ectopic pregnancy can be quite dangerous, the emergency room is a good place to diagnose it, as long as medical personnel is well trained and knowledgeable. Women with ectopic pregnancy who are monitored in the emergency room can receive robust emergent treatment if necessary.

If you are worried about finding your gestational sac on ultrasound, you likely have many questions. Talk to your doctor and if you don't understand something, ask again. Wondering about a pregnancy can be heart-wrenching. Reach out to family and friends who can walk this journey with you.

Sources

Goettler, S., and R. Zanetti-Dallenbach. Heterotopic Pregnancy. The New England Journal of Medicine. 2016. 375(29):1982.

Heaton HA. Ectopic Pregnancy and Emergencies in the First 20 Weeks of Pregnancy. In: Tintinalli JE, Stapczynski J, Ma O, Yealy DM, Meckler GD, Cline DM. eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8e. New York, NY: McGraw-Hill; 2016. 

Young, L., Barnard, C., Lewis, E. et al. The Diagnostic Performance of Ultrasound in the Detection of Ectopic Pregnancy. The New Zealand Journal of Medicine. 2017. 130(1452):17-22.

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