Very often parents are told by doctors that their twins are identical when they are in fact fraternal, and vice versa. A 2004 survey among members of the American College of Obstetricians and Gynaecologists found that 81 percent of doctors thought that twins who gestate with separate placentas are fraternal. In fact, 25 to 30 percent of identical twins have separate placentas and amniotic sacs.
In a new British study, researchers interviewed 1,302 parents of same-sex twins who had been told by health care professionals whether their children were fraternal or identical. Based on parental questionnaires and DNA analysis where available, the researchers classified 651 of the pairs as identical and 621 as fraternal. For 30 pairs, there was not enough information to decide.
They found that 191 couples — 14.7 percent — were misinformed about their babies, with 179 parents of identical twins mistakenly told that their twins were fraternal and 12 parents of fraternal twins told they were identical.