It seems unlikely that a singleton pregnancy was always present by ultrasound and later developed into a twin pregnancy. This is because ultrasound is primarily an ultrasound visualization to understand what is going on in the uterus. In the case of a twin pregnancy, even in the early stages of pregnancy, it is not detected as a twin pregnancy. After the second trimester, the two fetal heads are clearly within the uterine cavity and will be understood very clearly. So it is really unlikely that if you have been having a single fetus through ultrasound, you end up with a twin pregnancy before delivery. The difference between a twin pregnancy and a singleton pregnancy is very significant when you keep going to the hospital throughout the pregnancy and you will hear two fetal heartbeats during the labor and delivery. And between 23-28 weeks of pregnancy, a 4D ultrasound will be done, and under the 4D ultrasound you will see two fetal heads with multiple limbs, so this is unlikely to happen.