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 1,100.
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 doctor may order a follow-up ultrasound in a few days or a week.
If the hCG level is higher than 1,500 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 doctor 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.
Source:
American Pregnancy Association, "Concerns Regarding Early Fetal Development." Oct 2008. 16 Dec 2008.
