Current methods of fetal genetic testing typically involve obtaining samples of amniotic fluid, placenta, fetal blood or, rarely, other fetal tissues or fluids. The invasive techniques required for obtaining fetal samples (eg, amniocentesis, chorionic villus biopsy, fetal umbilical vessel venipuncture, fetoscopy-guided biopsy) place the fetus at risk of injury or death. Therefore, development of accurate, safe, rapid, noninvasive tests for prenatal diagnosis is an area of active investigation.
Fetal genetic material can be found in the maternal circulation, raising the possibility of using maternal blood to diagnose fetal disease. Intact fetal cells can be identified in maternal blood, but are not a reliable source of fetal genetic material because these cells are extremely rare [1,2] and may persist for years after prior pregnancies . By comparison, fetal "cell-free (cf)" nucleic acids not contained within cell membranes (cfDNA and cfRNA) are plentiful in the maternal circulation and unique to the current pregnancy. Thus, they have great potential for use in prenatal diagnosis. It is possible to noninvasively sequence the entire fetal genome [4-6].
Where do cell-free nucleic acids come from?
●DNA — Both the mother and the fetal-placental unit produce cfDNA. The primary source of non-maternal cfDNA in the maternal circulation is thought to be apoptosis of placental cells (syncytiotrophoblast), while maternal hematopoietic cells are the source of most maternal cfDNA [7-9]. There is some evidence that apoptosis of fetal erythroblasts also generates fetal cfDNA, which can cross the placenta and enter the maternal circulation [7,10,11]. Because placental nucleic acids are highly representative of fetal nucleic acids, we will use the term fetal cfDNA throughout the rest of the topic.
●RNA — Both fetal and placental cells contribute cell-free messenger RNA (cfmRNA) to the maternal circulation [12,13].
When are fetal cell-free nucleic acids found in maternal blood?
●DNA — Fetal cfDNA can be isolated from maternal blood as early as the fifth postmenstrual week, and almost always by the ninth postmenstrual week . The relative concentration of fetal cfDNA increases modestly (0.1 percent per week) with gestational age until approximately 21 weeks, and then increases rapidly (1 percent per week) until term .