When Does the Gestational Sac Become Visible on an Ultrasound?

The gestational sac is usually detected at 5 weeks.

Human embryo in amniotic sac at ninth week of gestation
Human embryo in amniotic sac at ninth week of gestation. Getty Images/DEA/L. RICCIARINI

In viable pregnancies, a transvaginal ultrasound should be able to detect the gestational sac by 5 weeks, gestational age or hCG level when it has reached about 1500 to 2000.

If it has been 5 weeks since the last menstrual period but the hCG level has not been measured, there is a possibility that seeing no gestational sac means that the woman ovulated late and the pregnancy has not yet reached 5 weeks gestational age.

The physician may order a follow-up ultrasound in a few days or a week.

If the hCG level is higher than 1500 to 2000 and the gestational sac is not visible, the doctor may diagnose ectopic pregnancy. If there are no hCG measurements or if the hCG level is lower, the physician will probably order a follow-up ultrasound or continued monitoring of the hCG level. It is also possible that the explanation is a very early miscarriage, aka chemical pregnancy, meaning that the 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 ultrasound is done in early pregnancy is to detect an intrauterine pregnancy and rule out 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 necessarily 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 where 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. Remember that the fallopian tube 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 and cause 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. 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 heterotopic pregnancy.

Ultimately, women who receive fertility treatments and in vitro fertilization should be carefully screened for heterotopic pregnancy.

However, in women without fertility treatment, the presence of an intrauterine pregnancy probably means that no ectopic pregnancy is present.

A Word From Verywell

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

Sources:

American Pregnancy Association, (August 2015). Concerns Regarding Early Fetal Development

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. 

Continue Reading