A third of all twins are identical ie they share the same genes and physical features.
Identical twins occur when one egg is fertilised by a sperm and then divides to form two separate embryos.
If the sonographer spots at your first scan that one of your babies is a boy and the other a girl, it will obviously confirm that your twins are non-identical.
One obvious sign of identical twins is the babies sharing a chorionic sac. But they could still be identical even if they have separate sacs.
Hospitals usually refer to separate placentas rather than chorionic sacs. Some, mistakenly, believe that two placentas means non-identical twins while a single placenta means your twins will be identical.
This is not, in fact, the case. It is possible for two placentas to become fused together or even just appear so close together that they look like a single placenta during your scan.
Under these circumstances, it is only after the birth that your hospital can confirm for sure whether your twins are identical by examining the membranes in the placenta.
Visit www.tamba.org.uk for more information. Visit their website to download Tamba’s free Healthy Multiple Pregnancy Guide for all the information you need about your multiple pregnancy.