Antenatal staff often misinform parents expecting same-sex twins based on evidence from ultrasound scans, a new study suggests.
According to the research, almost one in seven couples expecting same-sex twins are wrongly informed about whether their babies will be identical.
It is often believed that identical twins share one placenta, but evidence shows that 25% to 30% of identical twins have two placentas.
Researchers at University College London (UCL) gathered data from more than 1,500 women in England and Wales who gave birth to same-sex twins in 2007.
In 1,302 cases, parents had been told whether their twins were identical based on whether scans showed there were one or two placentas visible. DNA samples taken from the babies showed that 27.5% of parents of identical twins mistakenly believed their twins were non-identical.
Meanwhile, 2% of couples expecting non-identical twins had been wrongly told that their babies were identical.
More than a third (38%) of parents had been told that their twins were identical because an antenatal scan showed that the babies shared a placenta. Another 62% had been informed that their twins had two placentas and were therefore non-identical.
The findings have been published in BJOG: An International Journal of Obstetrics and Gynaecology. BJOG deputy editor-in-chief John Thorp said: “Additional training may be required for health professionals to avoid giving out the wrong information to parents.”