When an early pregnancy ultrasound finds no gestational sac, there are a few possible explanations:
- It is too early for the gestational sac to be visible
The gestational sac should become visible on a transvaginal ultrasound sometime before 5 weeks of gestational age, or by the time the hCG level has reached 1100. Before that, even in a viable pregnancy, there is not going to be a visible gestational sac on an ultrasound.
If the pregnancy is definitely past 5 weeks, or the hCG level is higher than 1100, a finding of no gestational sac is more likely to indicate a problem. 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 would almost certainly show a gestational sac in the uterus, and hCG levels should be rising.
- The pregnancy is ectopic
Whenever there is no gestational sac visible in the uterus, there is a possibility of ectopic pregnancy, which can become life-threatening or fatal in some cases if left untreated. Any time an ultrasound shows no gestational sac, the doctor will probably want to rule out ectopic pregnancy, especially when a woman has definite symptoms of 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 should definitely be visible if the pregnancy was located in the uterus. It is possible to have an early stage ectopic pregnancy without having clear symptoms of tubal pregnancy, and these pregnancies are not always medical emergencies when detected early.
- The pregnancy is miscarrying or has already miscarried
Especially if a woman had early pregnancy bleeding or other miscarriage symptoms leading up to the ultrasound, a finding of no gestational sac may mean that she had a very early miscarriage (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 miscarriage.
If You Have Not Yet Been Diagnosed with AnythingYou may be feeling very concerned and anxious, and perhaps even frustrated that you can't get a straight answer on what's going on. But the truth is that it can be difficult for doctors to determine right away which of the above explanations is behind a pregnancy with no visible gestational sac. 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 aren't sure whether it's 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/or monitoring of your hCG levels, which should give you a clear answer. The wait can be difficult but may be necessary for your doctor to be sure about the diagnosis.
American Pregnancy Association, "Concerns Regarding Early Fetal Development." Oct 2008. Accessed 19 Dec 2008.
Bree, RL, M Edwards, M Bohm-Velez, S Beyler, J Roberts, and EB Mendelson, "Transvaginal sonography in the evaluation of normal early pregnancy: correlation with HCG level." American Journal of Roentgenology 1989 153:75-79.
Pregnancy of Unknown Location: Management. The Royal Women's Hospital. Accessed 19 Dec 2008. http://www.thewomens.org.au/PregnancyofUnknownLocationPULManagement