I have my first ultrasound at 8 1/2 weeks and only had one baby. Then today I went for another…

I have my first ultrasound at 8 1/2 weeks and only had one baby. Then today I went for another ultrasound at 22 weeks. The doc just told me that I started out with twins but the other didn't take but I still got to go to the specialist to get another ultrasound just in case dose anyone know if theirs a name for this happening to women? Thank for any help..