After six weeks, the small white gestational ring is so characteristic that failure to identify it raises doubts about the pregnancy. By careful scanning, distinct echoes from the embryonic poles can be demonstrated within the gestational ring by seven weeks after the last period. Fetal heart must be observed by the eighth week, or the pregnancy is in serious doubt. Vaginal ultrasound is better at detecting very early pregnancy, so if the next one is questionable, I would seek that option. A small probe is placed in the vagina and the ultrasound is done in this manner.
Nothing goes through the cervix, however.
Take heart, it is too early to worry, but ask about the gestational sac next time as well. I hope all goes well for you. Watch this report about showing affection to boys. Find out here. You should not rely on this information as a substitute for personal medical attention, diagnosis or hands-on treatment.