When Does the Gestational Sac Become Visible on an Ultrasound?

The gestational sac is usually the first sign of pregnancy on ultrasound. When correlated with HCG levels, the presence of a gestational sac can be very helpful in detecting ectopic or heterotopic pregnancy. It can also be used to determine if you have experienced a very early miscarriage. In viable pregnancies, transvaginal ultrasound should be able to detect the gestational sac by 5 weeks gestational age.

Seeing Gestational Sac on Ultrasound

The main reason 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 always to determine viability and fetal age, though will be determined if intrauterine pregnancy is detected.

In early pregnancy, a transvaginal ultrasound is much more accurate than an abdominal ultrasound. If a transvaginal ultrasound is used, the sac may be visible at about 5 weeks. At this time the average diameter of the sac is 2 to 3 millimeters. At around 5.5 weeks, the yolk sac can be seen inside the gestational sac. An abdominal ultrasound is generally less sensitive and may not detect the gestational sac until a week or later in the pregnancy.

Not Seeing Gestational Sac on Ultrasound

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.

There are a few other reasons that a gestational sac may not be detected.

Ectopic Pregnancy

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.

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.

The tube can rupture or burst, which can cause internal bleeding and shock if it is left untreated. This is why it's very important that ectopic pregnancy is ruled out.

Many ectopic pregnancies are diagnosed in the emergency room using bedside ultrasound. As long as the medical personnel is well trained and knowledgeable, the emergency room can be a good place for the diagnosis to be made, as ectopic pregnancy can be dangerous.

Women with ectopic pregnancy who are monitored in the emergency room can receive robust emergent treatment if necessary.

Chemical Pregnancy

It is also possible that a very early miscarriage (chemical pregnancy) occurred. In this scenario, the pregnancy stopped developing before the gestational sac became large enough to be seen on ultrasound.

Rare Complications

In the vast majority of cases, the detection of intrauterine pregnancy means that no ectopic pregnancy is present. However, in rare cases, an intrauterine pregnancy and an ectopic pregnancy may occur at the same time.

This phenomenon is called a heterotopic pregnancy and is essentially a twin pregnancy with one fetus implanted in the uterus and the other in the fallopian tubes. 

Research has found that the risk of ectopic or heterotopic pregnancy may be increased when pregnancy is achieved through in vitro fertilization. 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

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.