Prenatal diagnosis enables the diagnosis of a broad spectrum of chromosomal abnormalities, gene disorders, X-linked conditions, neural tube defects, and infections to be made before the birth of the fetus. The various invasive prenatal diagnostic tests are amniocentesis, chorionic villus sampling, and fetal blood sampling or cordocentesis.
Amniocentesis is an invasive technique. This technique removes amniotic fluid from the uterine cavity using a needle. This procedure is performed transabdominally and under ultrasound guidance by a trained obstetrician. It was first performed for the diagnosis of genetic diseases (sex determination) of the fetus by Fuchs and Riis in 1956. It is performed for diagnostic and therapeutic purposes, including for diagnostic evaluation in the form of chromosomal, biochemical, histopathological, and microbial assessments. When performed as a therapeutic procedure, it is done to reduce the volume of amniotic fluid in patients with polyhydramnios.
The amniotic fluid obtained consists of fetal exfoliated cells, transudates, fetal urine, and lung secretions. Amniocentesis can be performed from 15 weeks of gestation to delivery, with an attributable risk of loss in experienced hands of 0.13% in singletons. Earlier the attributable risk is greater than chorion villus sampling (CVS), which provides similar data from 11 to 15 weeks. There is an increased risk of amnionic fluid leakage of 1 to 2%, most of which corresponds to decreased activity in days and fetal demise, which is usually much rarer than the risk of demise attributable to the indication for the procedure, except in low-risk women with no maternal risks indicating the procedure, and talipes equinovarus, presumably from oligohydramnios.
Counseling of the couple is necessary regarding the procedure's indications, risks, benefits, and limitations. This can be complex, as the individual circumstances leading to risk, including maternal age, parental family history, maternal serum screening, sonographic signs of chromosomal and other problems, and population history, can vary significantly. The benefit of possible pregnancy termination also varies depending on the patient's education, religious and ethical preferences, and, at present, state law.
Chorionic Villus Sampling
It is a prenatal invasive procedure and is done under ultrasound guidance. This procedure uses ultrasonography to guide the catheter or needle into the chorion frondosum. It is done abdominally and is followed by tissue aspiration (chorionic villi) for genetic or chromosomal analysis with a syringe containing tissue culture media. It is done in the first trimester for prenatal diagnosis between 10 to 14 weeks. Depending on the position of the uterus and bladder, the patient's gestational age, and placental localization, it can be performed transabdominally or transcervically.
The safer and earlier termination of pregnancy is possible as karyotype results are available within 7 to 10 days, although placental mosaicism is a risk for a false diagnosis or reassurance. It is indicated in chromosomal and genetic disorders. The samples collected are sent for DNA analysis. It is not performed in vaginal bleeding, in cases of cervical abnormalities, and severe infections. The major complications involved in this procedure are limb reduction defects from earlier procedures, chromosomal abnormalities present in the extraembryonic tissue, which are not found in the fetal tissue, intrauterine infections, membrane rupture, and fetal loss. The attributable risk in trained hands is similar to amniocentesis. Both procedures assume direct ultrasound guidance for a safe procedure.
Fetal Blood Sampling or Cordocentesis
It is the technique in which, under ultrasound guidance, fetal blood sampling is performed through the maternal abdomen. It is usually performed after 18 weeks after visualization of cord insertion. As the lumen of the cord at earlier weeks of gestation is narrow, it is considered safer to perform at 20 to 28 weeks of gestation. The blood sample is sent to the laboratory for hematological, immunological, and biochemical analysis. The results are obtained within 24 to 72 hours. There is an increased risk of fetal loss, which is comparatively higher (1 to 3% attributable risk) compared with other invasive procedures. The benefits include conversion to fetal transfusion, which can be life-saving. This is commonly indicated in maternal blood group sensitization from a transfusion or prior or current pregnancy sensitization to fetal cells from delivery or miscarriage of fetomaternal hemorrhage, suspected in the case of aplastic anemia from fetal Parvovirus B19 infection with the maternal acquisition of "Slapped face fever" - more common in teachers, parents, and childcare workers.
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