First trimester ultrasound is often done using a vaginal approach. Ultrasound also is invaluable to guide interventional procedures such as amniocentesis. The sensitivity of an ultrasound exam depends on a number of factors, such as the size and position of the fetus, the body habitus of the mother, the type of equipment used, and, most importantly, the skill and experience of the operator. As mentioned elsewhere, dating during this stage of pregnancy tends to be less accurate because biological variability is greater. The accuracy of ultrasound for dating a pregnancy depends on at what point during the pregnancy the ultrasound is taken.
Pregnancy dating is most accurate during the first half of pregnancy.
Measurement of the sac at five to seven weeks gives an accuracy of plus or minus ten days. Later in pregnancy, the accuracy declines, and may be plus or minus twenty-one days. I haven't found many estimates of exactly how accurate ultrasound is at detecting disabilities, but there are both false negatives and false positives. The accuracy will vary depending on the experience of the person doing the ultrasound. The accuracy also varies with which condition is being detected.