The information in this walkthrough is specific to ultrasounds done in early pregnancy, particularly in the early first trimester before seven weeks of pregnancy. As the pregnancy progresses, the ultrasound becomes more and more accurate for determining the viability of a pregnancy.
If an ultrasound in the second or third trimester shows that a baby has no heartbeat, this is considered conclusive for diagnosing a missed miscarriage or impending stillbirth.
Sources: American Pregnancy Association, "Concerns Regarding Early Fetal Development." May 2007. Accessed 26 May 2008. Savitz, David A., James W. Terry, Nancy Dole, John Thorp, Anna Maria Siega-Riz, Amy H. Herring, "Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination." American Journal of Obstetrics & Gynecology 2002. Accessed 26 May 2008.
