A new algorithm will be written to analyze the serial research blood samples to return a fetal fraction of each twin, and identify which twin is the DT verses the LT. The fetal fraction measurement will be a percentage, and it will be measurable, or not measurable for each sample.

Each complete participant in the study will have multiple maternal blood samples collected, and a child buccal swab. This outcome will use genetic analysis to reflect for each participant if the algorithm correctly or incorrectly identified the LT versus the DT. Accuracy, defined as the number of VT pairs correctly identified as the LT and DT, divided by the total number of VT pairs, will be calculated.

If a sufficient number of trisomy 21/18/13 cases are observed among any of the cases, assess the accuracy of the updated algorithm to correctly determine whether the LT or DT is aneuploidy. Point estimates and 95% confidence intervals for the specificity for the aneuploidy algorithm.