It is generally accepted that an intrauterine pregnancy should be seen on transvaginal ultrasound with a B-hCG level of 1000-2000 mIU/mL, and on transabdominal ultrasound with a level of 3000-5000 mIU/mL; this is called the discriminatory zone (Bree et al). If an intrauterine pregnancy (IUP) isn’t seen at this time, you should be suspicious of an ectopic pregnancy or non-viable intrauterine pregnancy.
However, there has been some debate on the utility of the discriminatory zone, with evidence showing the chance of a viable IUP developing later on despite no IUP detected below the discriminatory zones.