Since the introduction of ultrasound in the 1970s, it has become increasingly clear that many pregnancies start off with more than one embryo, and this is visible on the first ultrasound scan. When two or more embryos are visible on the first scan but one or more are no longer visible on a later scan, this is known as "Vanishing twin" phenomenon. Research carried out in the 1990s suggests that about 15% of pregnancies are probably affected. The surviving child has since 2003 been known as a "wombtwin" survivor. | 
