Background: Transvaginal sonography is used to assess the viability of first-trimester pregnancy. Significant data regarding normal and abnormal ultrasonographic measurements of the yolk sac (YS). The study aimed to know pregnancy outcome by assessing the role of ultrasonographic evaluation of yolk sac diameter and its shape in the first trimester. Patients and Methods: This study was conducted at the Obstetrics and Gynecology Department, Zagazig University Hospitals on 72 pregnant women in their first trimester of pregnancy. All pregnant women in their first trimester of pregnancy referred by the Department of Obstetrics for transvaginal sonography. All pregnancies with an abnormal appearing yolk sac were re-evaluated by sonography after two weeks. Results: The mean age was 26.9±3.9 years. It was found that there was no significant relationship between maternal age and pregnancy outcome. Regarding the absence of the yolk sac. It was not visualized in three cases, and all of them aborted later. Total mean YSD ± S.D was 3.7±1.8 mm, seen in GA from 6-9th weeks and there was no significant difference between the studied groups as regard YSD. Regarding normal YSD with the abnormal outcome, three cases with a normal YSD and regular morphology aborted after the 8 th week of gestation. Regarding large-sized YSD, it was found in three cases; two cases ended in a missed miscarriage at 8th week with YSD 8.4 mm and 8.2 mm respectively. In contrast, the third one with YSD 6.8 mm finally continued beyond 20 weeks with no demonstrable fetal anomalies. Regarding small-sized YSD; it was found in one case which ended in missed abortion at 7th week. Regarding the shape of the YS, an echogenic YS was detected in one case. Conclusion: The yolk sac diameter and shape as a sole parameter is a poor predictor for pregnancy outcome beyond 20 weeks and may be related to early pregnancy loss only. If any abnormality is observed, close follow up with ultrasonography is recommended for these pregnancies.