The earliest an ultrasound can be used when pregnant is about 5 weeks, according to BabyMed. The gestational sac can be seen via a trans-vaginal ultrasound starting at this time.
During pregnancy the earliest a doctor can do an ultrasound to see anything is between 5 to 10 weeks of gestation. According to BabyMed, a trans-vaginal ultrasound can reveal the gestational sac at this time. The gestational sac surrounds the embryo and is seen even before the embryo can be seen on an ultrasound, which is at about 6 weeks gestation.
Most doctors don't require an ultrasound this early, but will instead schedule the first ultrasound between 10 to 13 weeks plus 6 days of gestation, as stated on BabyCenter. This is the dating ultrasound to help the doctor estimate the due date of the baby more accurately. The early ultrasound is only requested if the mother has had a previous miscarriage or bleeding early in the pregnancy. Then an early ultrasound can help reassure the mother about the health of the embryo and give the doctor vital information about the pregnancy.
A second ultrasound is usually carried out later in the second trimester between 18 to 20 weeks. This more detailed ultrasound gives the doctor a good look at the developing fetus and can also help determine the baby's gender.