There is no specific value that defines low fetal free DNA, this is because the value that defines fetal free DNA levels rises slowly as the weeks of pregnancy increase, and the value that defines low DNA levels varies from time to time. Fetal free DNA refers to the fact that about 1%-5% of the DNA in the peripheral blood serum of a pregnant woman is of fetal origin and is called fetal free DNA. Fetal free DNA is exchanged through the placental circulation and can be present in the peripheral blood of pregnant women. Fetal free DNA exists stably in the peripheral blood of pregnant women in the form of small fragments of 75-205 bp, accounting for 5%-30% of all free DNA in plasma, so the concentration of fetal free DNA in the range of 5%-30% is normal. Pregnant women usually have fetal free DNA genes in their blood after 12 weeks, and the higher the gestational week, the higher the level, but there is no specific value that defines a low level of free DNA, because this standard is always changing. If the DNA concentration is low, it is necessary to analyze according to the actual situation, because there are many factors that can affect the concentration of DNA, such as the second week of pregnancy, or less than 12 weeks on the examination, the process of blood collection and degradation, and pregnant women are too thin or too fat, will affect the concentration of DNA.