In Denmark about 1400 twin pairs are born every year. Just like all other births they are reported to the Medical Birth Register of Health and by this means the Danish Twin Registry can keep all information updated and revised.
The Danish Twin Registry will usually not contact the parents during the first years. Not until the twins are about 6-8 years old, the register will address the parents and apply for some basic information about the children and their first years of life. It is of course entirely voluntary for the parents to reply, but for scientific purposes it is of great importance that as many as possible do participate.
One of our main issues is whether the twins are monozygotic or dizygotic. In the University Hospitals in Odense and Skejby parents of newborn twins of the same sex, will be offered a test that can tell whether the twins are mono- og dizygotic. The blood sample is taken routinely in connection with the other samples, and it is free of charge.
For all other same-sex twins, we usually use some questions about the twins' similarity to each other to find out whether the twins are mono- og dizygotic:
- Do they look alike?
- Can anyone tell the difference?
- Do they often get mistaken for each other?
- Do they have the same hair and eye color?
This method has proved to give the right answer for about 95% of all cases.