I guess it could be possible, but it would be a very rare and freaky event. Even as early as 5 weeks they can detect twin pregnancies, and if you’ve had more than one, I’d say your chance of you carrying and undetected 2nd pregnancy would be impossible, unless you had the most inept u/s technician in the world.

Also, you would be measuring much larger around your belly if you had a twin pregnancy. Doctors should be checking the distance between your pubis and the top of your uterus at each appointment. If this measurement is outside of the normal range, it can indicate a problem, like too much fluid in the sac, so the doctors keep a close eye on it. Women who are carrying twins have larger measurements. If you were carrying twins, your doctor should have noticed many, many weeks ago that you are measuring big, and he/she would have performed an additional u/s to determine the cause of the discrepency. So it sounds like you’ve got a singleton in there. Good luck with delivery and I hope your daughter is happy and healthy!