Most accurate dating pregnancy ultrasound
Kmom can't emphasize the importance of reading BOTH FAQs strongly enough!
If you need more detail about statistics, interpretation of results, rates of 'false-positives', etc., then be sure to research the many websites devoted to prenatal testing online.
There are other types of ultrasound, and new technologies being developed all the time.
However, information on these is beyond the scope of this FAQ. Vaginal ultrasound is used for very early pregnancy, and sometimes for heavier women with more abdominal fat.
Katz Rothman is by no means arguing against the use of prenatal testing; she actually presents a number of compelling reasons to consider it.
Her writing is a fair and balanced look at the intricacies and difficulties of this issue.