The gestational sac encloses the developing baby and contains amniotic fluid. If it’s not visible by around 5 weeks gestation, it may mean that the pregnancy is not viable or a miscarriage has already happened. However, you may have your timing off and it’s just too early to see, in which case, a follow-up ultrasound will likely be ordered. Learn more about when the gestational sac is usually seen and the different reasons why it can’t be found on an ultrasound in early pregnancy.
When Is a Gestational Sac Visible?
A gestational sac is one of the first signs of pregnancy that can be seen on a transvaginal ultrasound. The transvaginal ultrasound is generally used because an abdominal ultrasound is much less accurate this early in pregnancy. When identified on the ultrasound, the diameter of the sac is around 2 to 3 millimeters and is seen as a white rim around a clear center in your uterus. If you are also having quantitative pregnancy tests (serum hCG tests) the gestational sac usually becomes visible when hCG levels have reached 1500 to 2000. The next step on ultrasound is usually the appearance of the yolk sac within the gestational sac. The yolk sac functions as nourishment for the developing embryo and can usually be seen by 5.5 to 6 weeks gestational age on a transvaginal ultrasound.
Common Reasons for No Gestational Sac
Having a gestational sac does not say much about the health of your pregnancy, nor does it say whether an embryo is present or not. The gestational sac is essentially the sac that surrounds the baby (when it becomes visible) and contains amniotic fluid. If a gestational sac is not seen on an early pregnancy transvaginal ultrasound by around 5 weeks gestational age, there are several things that could be occurring.
Too Early in the Pregnancy
The gestational sac is typically visible on a transvaginal ultrasound somewhere between 3 to 5 weeks of pregnancy, or by the time the hCG has reached 1500 to 2000. Before that, even in a viable pregnancy, there is not going to be a visible gestational sac on an ultrasound.
It is too early in the pregnancy.The pregnancy is ectopic.The pregnancy is a miscarriage.
But when there’s no confirmation of hCG levels or any definite evidence of the dating of the pregnancy, the pregnancy might still be in very early stages. In this case, a follow-up ultrasound is recommended. If the pregnancy is definitely past 5 weeks, or the hCG level is higher than 2000, a finding of no gestational sac is more likely to indicate a problem.
Ectopic Pregnancy
Whenever there is no gestational sac visible in the uterus, there is a possibility of ectopic pregnancy. This can be frightening, but keep in mind that the absence of a gestational sac is more likely to be related to wrong dates. Even if an ectopic pregnancy is detected, these can be managed well when caught early. An ectopic pregnancy is still a possibility even if you don’t have symptoms of an ectopic pregnancy. A doctor may be able to diagnose ectopic pregnancy and recommend treatment without a follow-up ultrasound if the hCG levels are high enough that the gestational sac would definitely be visible if the pregnancy was located in the uterus. These pregnancies are not always medical emergencies when detected early, and treatment options are not always surgical.
Miscarriage
If you have experienced early pregnancy bleeding or other miscarriage symptoms, a finding of no gestational sac may mean that you’ve had a very early miscarriage (also called a chemical pregnancy) or that the pregnancy tissue has already left the uterus. If there are falling hCG levels along with a finding of no gestational sac, the diagnosis is almost certain to be a miscarriage. As with an ectopic pregnancy, there are different treatment options available if you have a miscarriage.
What to Do If There Is No Gestational Sac
It can be difficult for doctors to determine right away which of the above explanations is behind a pregnancy with no visible gestational sac. Therefore, it is completely natural to feel concerned and anxious, and perhaps even frustrated. You may be told (or see on medical forms) that you have a “pregnancy of unknown location,” which simply means that the ultrasound did not show a gestational sac, and the doctors are not sure whether it is an ectopic pregnancy, a miscarriage, or a very early but otherwise normal pregnancy. Most likely, you will be asked to come back for a follow-up ultrasound and undergo monitoring of your hCG levels. Together, these repeat tests should give you a clear answer. The wait can be difficult but may be necessary for your doctor to be certain of your diagnosis.
