Cell-free DNA Testing

Fertilized eggs transform into morula in three days, and this stage of fetal development is known as early blastocyst. The early blastocyst enters the cavity of uterus and transforms into a blastocyst (which includes an inner cell mass and a trophoblast) around the 4th day post-fertilization. At around 6-7 days post-fertilization, part of the blastocyst (i.e. the trophectoderm) becomes the the placenta. Following this, some trophectoderm cells die and fetal DNA segments enter into the mother’s circulatory system.

The length of fetal DNA segments is usually around 150-200 base pairs (bp). These segments can be detected around the 18th day of pregnancy using Real Time Polymerase Chain Reaction (RT-PCR). According to a 2013 article published in the journal Prenatal Diagnosis, the concentration of fetal DNA in the mother’s blood is around 10.2% at week 10, and this increases by 0.1% each week between weeks 10 to 21. After this, the concentration of fetal DNA in the mother’s blood increases by 1% every week.