Background/Aims: In pregnancy complicated with type-1 diabetes mellitus (DM), information on the predictive value of yolk sac (YS) measurement and YS morphology on outcome has been scant. The aim of this study was to compare the size and shape of the YS using transvaginal sonography (TVS) in normal pregnancies and those complicated with well-regulated type-1 DM. Methods: Sixty healthy pregnant women and 60 pregnant women with type-1 DM underwent TVS. The correlation coefficients between YS and gestational age (GA), YS and crown-rump length (CRL), and YS and human chorionic gonadotropin (HCG) were calculated. Statistical analysis showed that the difference between the YS diameters for each gestational week among the groups studied was highly statistically significant. Statistical analysis of the results was performed with SPSS 10 software. Results: A highly statistically positive correlation was found between YS diameter and GA, CRL and HCG levels for both groups studied. The YS diameter in type-1 DM pregnancies was statistically significantly larger for GA than that of healthy pregnant women. The YS diameter in type-1 DM women was statistically significantly larger after 6 weeks of gestation and the HbA1c levels were >6% when compared to the YS dimensions in the group with HbA1c concentrations of <6%. Conclusion: A gross change in YS size may indicate or reflect significant dysfunction of the maternofetal transport system in early pregnancy which is complicated with type-1 DM.