How Early Can an Ultrasound Detect a Fetus?

The gestational sac is seen earlier than a fetus can be detected on an ultrasound. A gestational sac can be seen as early as the fifth week of pregnancy or three weeks after conception. If no gestational sac is seen, it could mean that the expectant mother ovulated late. If HCG levels are 1100, most doctors will order a repeat ultrasound in a few days. If HCG levels are higher and no gestational sac is seen, it could be an indication of an ectopic pregnancy, or a pregnancy that implants outside of the uterus.

Transvaginal ultrasound can be used to detect a fetus or gestational sac up to a week before traditional ultrasound. A transvaginal ultrasound is performed by inserting a lubricated wand into the vagina and performing the ultrasound internally. Transvaginal ultrasound can be used when the mother's bladder is empty, making it difficult to see the fetus, or when the fetus is too small to detect on traditional ultrasound.

A fetus can be detected as early as the sixth week of pregnancy. Many ultrasound technicians prefer to wait until at least that point in the pregnancy to do an ultrasound and many prefer waiting until the eighth week of pregnancy when more of the development of the fetus can be seen.

A fetal heartbeat appears on an ultrasound as a small flickering image. About six-and-a-half weeks into a pregnancy, most ultrasound technicians can clearly see the fetal heartbeat. Since a fetal Doppler may not be able to pick up the fetal heartbeat until later in the first trimester, seeing the heartbeat on the ultrasound screen can be very reassuring for an expectant mother.

If no gestational sac or fetus is seen on an early ultrasound, many doctors will choose to do another ultrasound in a few days to a week. If an expectant mother ovulated late, a fetus would not be visible during the time frame when it would be expected.