A missed twin abortion refers to the loss of one fetus in a multiple pregnancy while the other continues to develop. If your scan at 23 weeks indicates that you are actually only 17 weeks pregnant and there is evidence of a missed twin abortion, it means that one of the twins has stopped growing or has been absorbed by the body. This can be caused by various factors such as chromosomal abnormalities, placental issues, or maternal health conditions. Symptoms may include reduced fetal movement or no heartbeat for one of the twins. It is important to consult with your healthcare provider for further evaluation and guidance. They may recommend additional tests or monitoring to ensure the well-being of the remaining fetus and provide emotional support during this difficult time.