A genetic test that has made a splash in the field of prenatal diagnosis in the last decade was a landmark discovery in 1997 when researchers identified a fragment of the Y chromosome in male fetuses from the peripheral blood of pregnant women. Free fetal DNA fragments are now known to typically appear in the mother’s plasma at 5 weeks of pregnancy and disappear rapidly the day after delivery. Between 3% and 10% of the free DNA in the mother’s circulating plasma during pregnancy originates from the fetal placental unit, accounting for 3% to 6% of total maternal plasma DNA, with levels fluctuating widely, from 2% to 35%, and with maternal physiological changes.
The test procedure is performed by collecting peripheral blood (5-10 ml) from pregnant women at 12 weeks to 26+6 weeks of gestation, extracting free DNA, and using next-generation high-throughput sequencing combined with bioinformatic analysis to derive the risk rate of fetal chromosomal aneuploidy disorders. According to the current level of technological development, non-invasive fetal DNA testing is suitable for the screening of common fetal chromosomal aneuploidy abnormalities (i.e. trisomy 21, trisomy 18, trisomy 13) with an accuracy rate of over 99%.
Readers would prefer to know under what circumstances the test can be selected and under what circumstances it is not suitable. Please pay attention to this article for the introduction and analysis of the applicable, cautious and inapplicable groups of people.
I. Applicable groups
1. Pregnant women whose serological screening (also known as “Down syndrome screening” during early pregnancy, Down screening) shows a critical risk of common chromosomal aneuploidy (e.g. 1/1000 ≤ risk of Down syndrome < 1/270, 1/1000 ≤ risk of trisomy 18 < 1/350). 270 and 1/350 are the cut-off values for high risk of the two syndromes respectively, which are set by individual laboratories according to 5% and 10% false positives, and may vary slightly from hospital to hospital, but will appear in the Down syndrome screening report.
2. Patients who are not suitable for amniocentesis, such as pre-eclampsia, fever, bleeding tendency, untreated infection, maternal-fetal RH blood type incompatibility, placenta previa, etc.
3. Pregnant women who miss the best time for serological screening at gestational week or miss the time for routine prenatal diagnosis but wish to reduce the risk of trisomy 21, trisomy 18, trisomy 13 Down syndrome.
II. Cautious population
Pregnant women with the following conditions belong to the cautionary population, i.e. the screening effect of the test in this population has decreased to a certain extent, i.e. the detection rate of screening has decreased, the false-positive and false-negative rates have increased, or pregnant women who meet the indications for interventional prenatal diagnosis (amniotic fluid or cord blood puncture). This includes.
1.Pregnant women with high risk of serological screening (i.e. screening for Down’s syndrome), advanced age with expected age ≥ 35 years, and at risk for other chromosomal abnormalities can undergo direct prenatal diagnosis.
2, Pregnant women with <12 weeks gestation, the level of fetal free dna is too low and
3.Pregnant women with high weight (weight >100 kg).
4, Pregnant women conceived by IVF through in vitro fertilization-embryo transfer, if singleton conception after transferring two embryos, the possible residual material of the unconceived embryo may interfere with fetal free DNA in maternal blood; successful conception of twin pregnancies after transferring two embryos will also reduce the detection rate of screening, resulting in an increase in the false positive and false negative rates.
5. Pregnant women with dual chorionic twin pregnancies.
6. Pregnant women with combined malignant tumors.
7.Independent imaging screening for soft indicators, such as nasal bone loss, NF thickening, and other ultrasound abnormalities that may be associated with chromosomal aneuploidy.
Third, not applicable to the population
1. Pregnant women with a history of delivery of chromosomally abnormal fetuses.
2. One of the couple has clear chromosomal abnormalities (excluding normal chromosomal variants).
3. Pregnant women who have received allogeneic blood transfusion, transplantation, cell therapy or immunotherapy within 1 year, which may interfere with the results of non-invasive fetal DNA test.
4. Fetus with microdeletion-microduplication syndrome or other chromosomal abnormalities suspected by fetal imaging, e.g., multiple positive imaging screening soft indicators.
5. High risk group of various monogenic diseases.
Finally, pregnant women who wish to choose the non-invasive fetal DNA test program are reminded to provide the following information to the physician when applying for this test.
(1) Name of the pregnant woman, date of birth, weight at the time of blood collection, mailing address and contact number of the pregnant woman.
(2) Maternal history and number of fetuses.
(3) Provide the date of the last menstrual period.
(4) Provide the results of other prenatal testing, serological prenatal screening or prenatal diagnosis.
(5) whether the couple has chromosomal abnormalities and a family history of monogenic disease
(6) Whether assisted reproduction conception was performed
(7) Whether cellular therapy, allogeneic blood transfusion or whether the patient is a tumor patient, etc. has been performed.