Generally speaking, 2/3 of twins are fraternal and the other 1/3 are identical. It is especially difficult to perform genetic testing on fraternal twins as they possess different genotypes. Previous research has shown that, when a woman is pregnant with twins, the concentration of cell-free DNA in blood is 35% higher than for a single fetus, yet the concentration is lower for each individual fetus. In these cases, more accurate testing protocols are used, including:

• 1. Sequencing a greater amount of DNA to reduce statistical interference.
• 2. Balancing internal variance with external variance using a formula.

The SAFeRTM computational method by illumina can be used to test for chromosome abnormalities in twins when the concentration of cell-free DNA is low.