The Rare Occasions of a Hidden Twin. As with most pregnancy concerns, there are some very rare exceptions. Ultrasound provides a picture of the womb, but sometimes the picture can be misleading or misinterpreted. After twenty weeks, a second fetus should be clearly visible on ultrasound. The likelihood that there is another baby hidden in the womb is extremely minuscule. Chances are, you are not having twins if there is no evidence of multiples on ultrasound.