The discriminatory level of hCG refers to the level at which a gestational sac should always be seen on transvaginal US in a normal singleton IUP and has historically been suggested as 1,000 to 2,000 mIU/mL [12,13]. However, even a level of 2,000 mIU/mL has been found to be too low to exclude a normal IUP [14-16]. If there is no transvaginal US evidence of a gestational sac when a single serum hCG is 3,000 mIU/mL or higher, it is unlikely there will be a viable IUP [17]. For a hemodynamically stable patient with no sonographic evidence of an IUP or ectopic pregnancy, management decisions should generally not be made based on a single hCG level [16,17] . Follow-up hCG assay and US are usually appropriate in such a scenario. Definite intrauterine gestation The yolk sac, a thin-walled, spherical structure with an anechoic center, is the first sonographic feature that confirms an IUP [8]. It is usually visualized by transvaginal US in a gestational sac >8 mm in MSD [18]; however, in some normal pregnancies the gestational sac will be larger before a yolk sac is seen [19]. The embryo will initially appear as a thickened, linear echogenic structure at the edge of the yolk sac. With transvaginal US, the embryo is usually seen by about 6 weeks gestational age and by the time the gestational sac grows to a MSD of 16 mm; however, in some normal pregnancies the gestational sac will be larger before an embryo is seen [20]. Once an IUP is definitely established by US, various US findings may be seen that are associated with a nonviable IUP. These include bradycardia [23,24], small gestational sac compared to embryo [25], enlarged amniotic cavity [26], empty amniotic cavity [27], absence of cardiac activity with visualization of the amnion [28], and abnormal size or shape of the yolk sac [29]. However, these findings are not definitive for a nonviable IUP, and in these situations, follow-up US in correlation with serial quantitative serum beta hCG measurements is often useful.
First Trimester Vaginal Bleeding. The discriminatory level of hCG refers to the level at which a gestational sac should always be seen on transvaginal US in a normal singleton IUP and has historically been suggested as 1,000 to 2,000 mIU/mL [12,13]. However, even a level of 2,000 mIU/mL has been found to be too low to exclude a normal IUP [14-16]. If there is no transvaginal US evidence of a gestational sac when a single serum hCG is 3,000 mIU/mL or higher, it is unlikely there will be a viable IUP [17]. For a hemodynamically stable patient with no sonographic evidence of an IUP or ectopic pregnancy, management decisions should generally not be made based on a single hCG level [16,17] . Follow-up hCG assay and US are usually appropriate in such a scenario. Definite intrauterine gestation The yolk sac, a thin-walled, spherical structure with an anechoic center, is the first sonographic feature that confirms an IUP [8]. It is usually visualized by transvaginal US in a gestational sac >8 mm in MSD [18]; however, in some normal pregnancies the gestational sac will be larger before a yolk sac is seen [19]. The embryo will initially appear as a thickened, linear echogenic structure at the edge of the yolk sac. With transvaginal US, the embryo is usually seen by about 6 weeks gestational age and by the time the gestational sac grows to a MSD of 16 mm; however, in some normal pregnancies the gestational sac will be larger before an embryo is seen [20]. Once an IUP is definitely established by US, various US findings may be seen that are associated with a nonviable IUP. These include bradycardia [23,24], small gestational sac compared to embryo [25], enlarged amniotic cavity [26], empty amniotic cavity [27], absence of cardiac activity with visualization of the amnion [28], and abnormal size or shape of the yolk sac [29]. However, these findings are not definitive for a nonviable IUP, and in these situations, follow-up US in correlation with serial quantitative serum beta hCG measurements is often useful.