Early transvaginal ultrasound following an accurately dated pregnancy: the importance of finding a yolk sac or fetal heart motion.
Deaton JL, Honoré GM, Huffman CS, Bauguess P. Early transvaginal ultrasound following an accurately dated pregnancy: the importance of finding a yolk sac or fetal heart motion. Hum Reprod. 1997 Dec; 12(12):2820-3.