Tawfik, W. M. “Evaluation of Yolk Sac Size and Embryonic Heart Rate in First Trimester and Pregnancy Outcome”. International Journal of Reproduction, Contraception, Obstetrics and Gynecology, vol. 10, no. 8, July 2021, pp. 2975-80, doi:10.18203/2320-1770.ijrcog20212943.