ALMOST one in seven couples expecting same-sex twins are given the wrong information about whether their offspring are identical, research suggests.
Expectant parents can end up misinformed about their unborn children during ultrasound scans carried out during pregnancy.
Data from more than 1,500 families with same-sex twins born in 2007 in England and Wales was gathered for the study, published in BJOG: An International Journal of Obstetrics and Gynaecology.
It found that 1,302 parents had been told whether their twins were identical based on whether there were one or two placentas visible during antenatal scans.
It is commonly believed that identical twins share one placenta, even though evidence suggests 25pc to 30pc of identical twins have two placentas.
Researchers from University College London (UCL) took DNA samples from the babies to discover whether the predictions had been correct.
Overall, 651 twin pairs were found to be identical and 621 were non-identical.
Of the 1,302 parents, 191 (15pc) were misinformed about whether their child was identical.
As many as 27.5pc of parents of identical twins mistakenly believed their twins were non-identical, while 2pc of parents of non-identical twins mistakenly believed their twins were identical.
In total, 38pc of parents said they were told after an antenatal scan that their twins shared a placenta and were therefore identical, while 62pc of parents were told their twins were non-identical as they had two placentas.
The experts concluded: "The separateness of the placentas and amniotic sacs in utero can be determined during prenatal ultrasound scans and parents are often informed by health professionals about the significance of such observations for zygosity (whether twins are identical).
"However the primary purpose of these ultrasound scans is not to predict zygosity, they are clinically important in assessing the risk associated with ... twin pregnancies.
"Two separate placentas and amniotic sacs do not denote dizygosity, because if (identical) twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from that of (non-identical) twins."
Some of the families felt angry when they were told the truth about their children.
Jane Wardle, from the health behaviour research centre at UCL and co-author of the paper, said: "Finding out if your twins are identical or not is important to parents.
"However our data suggests that there may be a lack of knowledge among some health professionals about both identical and non-identical twins having two placentas.
"Parents have the right to the relevant information if it is available. However most parents will understand that it is not always possible to give a specific answer."
John Thorp, BJOG deputy editor-in-chief, added: "This paper highlights the uncertainty that can exist in finding out whether same sex twins are identical or not.
"Additional training may be required for health professionals to avoid giving out the wrong information to parents."