It has been previously suggested that abnormal yolk sac characteristics and slow embryonic foetal heart rate are associated with poor gestational outcome and these parameters can be used to assess foetal viability in first trimester of pregnancies. The purpose of present study was to examine the role of yolk sac characteristics and early embryonic heart rate (EHR) between 6 to 9 weeks of pregnancies and to study their associations with spontaneous abortions. This prospective observational study included 150 low risk healthy singleton pregnant women attending antenatal clinic in a tertiary hospital in early gestation. Transvaginal ultrasound was carried out to quantify yolk sac morphometry and using M mode, early embryonic heart rate was measured. 90% (135/150) of subjects satisfied normal yolk sac morphometry according to Nyberg criteria and 99.2% of them (134/135) had ongoing pregnancies. 15 patients had abnormally appearing yolk sac and 33.3% of them (5/15) subsequently aborted. Similarly 95.3% (143/150) who had normal embryonic heart rate (≥100 bpm) had successful outcome. Abnormal heart rate (<100 bpm) was detected in 7 pregnancies and majority of them (6/7, 85.7%) ended up in foetal demise. Yolk sac diameter between 2–5 mm showed sensitivity, specificity and accuracy of 93.3%, 83.3% and 92.9% for successful gestational outcome. The respective figures for embryonic heart rate (≥ 100 bpm) were 98.6%, 83.3% and 98.5%. The embryos with good yolk sac parameters and normal heart rate have high potential for survival. The information obtained by first trimester vaginal ultrasound may be used to prognosticate pregnancies that are complicated by previous bad obstetric history.