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EHRBBII. en DuBose TJ obgyn net asp page DuBose TJ Cunyus JA Johnson Embryonic Heart Rate and Age Diagn Med Sonography Schats Jansen CAM Waldimiroff JW Embryonic heart activity appearance and development in early human pregnancy Brit Obstet and Gynaecol Qasim SM Sachdev Trias et al The predictive value of first trimester