If you have an ultrasound during the first trimester and a gestational sac is seen, that’s a definite sign of pregnancy. The earliest visible sac can be seen at 4 1/2 weeks and in almost all normal pregnancies by 5 weeks along (obviously excluding ectopic pregnancies).
At 32- 35 days along, the first ultrasound evidence of pregnancy can be detected and an ultrasound during this time and the weeks following are most accurate in determining gestational age estimates.
An ultrasound early in the first trimester is done using a trans-vaginal technique, where a probe is placed directly into the vagina, rather than a transducer onto the abdomen. This way, the transducer can get a closer image of the uterus and is better at detecting a very early pregnancy.
more Pregnancy Signs