Invasive methods of prenatal diagnosis

Invasive methods of prenatal diagnostics are widely used to identify a large number of fetal diseases, including genetic diseases and chromosomal abnormalities (trisomies of 18 and 21 chromosome pairs, cri du chat syndrome, Duchenne muscular dystrophy, neural tube defects, congenital metabolic disorders, etc.), as well as to detect fetal abnormalities.

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Amniocentesis

Collection of amniotic fluid for biochemical, hormonal, immunological, cytological and genetic studies, allowing to judge the condition of the fetus. Indications for amniocentesis are: iso-serological incompatibility of the mother's and fetus's blood, chronic fetal hypoxia (post-term pregnancy, OPG-gestosis, extragenital diseases of the mother, etc.), determination of the degree of fetal maturity, antenatal sex diagnosis, cardiological examination in case of fetal malformations, microbiological examination.

Depending on the puncture site, transvaginal and transabdominal amniocentesis are distinguished. Transvaginal amniocentesis is recommended for pregnancy up to 16-20 weeks, transabdominal - after 20 weeks. The operation is always performed under ultrasound control, choosing the most convenient puncture site depending on the location of the placenta and small parts of the fetus.

In transabdominal amniocentesis, after treating the anterior abdominal wall with an antiseptic solution, the skin, subcutaneous tissue, and subgaleal space are anesthetized with a 0.5% solution of novocaine. At least 40 ml of amniotic fluid is required for the examination. The puncture site on the anterior abdominal wall is treated with an antiseptic and an aseptic sticker is applied. Transvaginal amniocentesis is performed through the anterior vaginal fornix, the cervical canal, or the posterior vaginal fornix. The choice of the insertion site for the puncture needle depends on the location of the placenta. After preliminary sanitation of the vagina, the cervix is fixed with bullet forceps, shifted up or down, depending on the chosen method, and the vaginal wall is punctured at an angle to the uterine wall. When the needle penetrates the uterine cavity, amniotic fluid is released from its opening.

The biochemical composition of the amniotic fluid is relatively constant. There are minor fluctuations in the concentration of mineral and organic substances depending on the gestational age and the condition of the fetus. The pH of the amniotic fluid correlates with that of the fetal blood obtained from the scalp. In full-term pregnancy, the pH of the amniotic fluid is 6.98-7.23. The most informative values for diagnosing fetal hypoxia are pH (less than 7.02), pCO2 (over 7.33 kPa), p02 (less than 10.66 kPa), potassium concentration (over 5.5 mmol/l), urea (7.5 mmol/l) and chlorides (over 100 mmol/l). One of the important indicators of metabolism in the amniotic fluid is creatinine, the concentration of which increases as pregnancy progresses and at the end of it is 0.18-0.28 mmol/l. Creatinine reflects the degree of maturity of the fetal kidneys, an increase in its level in the amniotic fluid is observed in fetal hypotrophy and late toxicosis of pregnancy. An increase in the protein content in the amniotic fluid may indicate hemolytic disease, intrauterine fetal death, anencephaly and other fetal developmental abnormalities. The glucose level in the amniotic fluid of 15 mg/100 ml and above is a sign of fetal maturity, below 5 mg/100 ml - its immaturity. In post-term pregnancy, the glucose concentration decreases by 40% due to a decrease in the glycogen content in the placenta due to dystrophic changes.

To diagnose hemolytic disease of the fetus, the optical density of bilirubin (ODB) in the amniotic fluid is determined. The ODB value is determined using a spectrophotometer at a wavelength of 450 nm. If ODB is below 0.1, the spectrophotometric curve is assessed as physiological.

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Cytological examination of amniotic fluid

In order to diagnose the degree of maturity of the fetus, a cytological examination of the amniotic fluid is carried out. The main source of the cellular composition of the amniotic fluid is the skin and epithelium of the urinary tract of the fetus. It includes the epithelium of the amnion, umbilical cord and oral cavity of the fetus. To obtain and examine the sediment, the amniotic fluid is centrifuged at 3000 rpm for 5 minutes, smears are fixed with a mixture of ether and alcohol, then stained using the Harras-Shore, Papanicolaou method or 0.1% solution of Nile blue sulfate, which stains the anuclear lipid-containing cells (a product of the sebaceous glands of the skin of the fetus) orange (the so-called orange cells). The percentage of orange cells in the smear corresponds to the maturity of the fetus: up to 38 weeks of pregnancy, their number does not exceed 10%, over 38 weeks - reaches 50%. To assess the maturity of the fetal lungs, the concentration of phospholipids in the amniotic fluid, especially the lecithin/sphingomyelin (L/S) ratio, is measured. Lecithin, a saturated phosphatidylcholine, is the main active principle of surfactant. L/S ratio values are interpreted as follows:

• L/S = 2:1 or more - lungs are mature; only 2% of newborns are at risk of developing respiratory distress syndrome;
• L/S = 1.5-1.9:1 - the probability of developing respiratory distress syndrome is 50%;
• L/S = less than 1.5:1 - in 73% of observations, the development of respiratory distress syndrome is possible.

In everyday practice, a qualitative assessment of the lecithin and sphingomyelin ratio is used (foam test). For this purpose, 3 ml of ethyl alcohol is added to a test tube with 1 ml of amniotic fluid and the test tube is shaken for 3 minutes. The resulting foam ring indicates fetal maturity (positive test), the absence of foam (negative test) indicates immaturity of the lung tissue.

Amniotic fluid testing for the purpose of diagnosing congenital malformations is usually performed at 14-16 weeks of pregnancy. Fetal cells contained in the amniotic fluid and used for genetic research are grown in tissue culture. Indications for amniocentesis in this case are:

• the woman is over 35 years old (taking into account the high risk of developing trisomy 21 pairs of chromosomes);
• presence of chromosomal diseases in children born earlier;
• suspected X-linked disease in the mother.

Complications of amniocentesis: premature rupture of membranes (more common with transcervical access), injury to fetal vessels, injury to the mother's bladder and intestines, chorioamnionitis; less common - premature birth, placental abruption, fetal injury and umbilical cord injury. However, due to the widespread use of ultrasound monitoring, complications of amniocentesis are extremely rare.

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Chorionic villus sampling

An operation aimed at obtaining villous chorion cells for fetal karyotyping and determination of chromosomal and gene abnormalities (including determination of hereditary metabolic disorders). Samples are taken transcervically or transabdominally between 8 and 12 weeks of pregnancy under ultrasound scanning control. Complications of chorionic villus biopsy may include intrauterine infection, bleeding, spontaneous abortions, and hematomas. Later complications include premature birth, low birth weight (<2500 g), and fetal malformations. Perinatal mortality reaches 0.2-0.9%.

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Cordocentesis

Cordocentesis (obtaining fetal blood samples by puncturing the umbilical vein) is performed for fetal karyotyping and immunological studies. Relative contraindications for cordocentesis are oligohydramnios, polyhydramnios, and poor fetal positioning. Potential complications (1-2%): chorioamnionitis, rupture of membranes, Rh immunization, fetal bleeding, umbilical cord vascular hematoma, intrauterine fetal growth retardation.

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Fetal surgery

With the improvement of ultrasound and invasive prenatal diagnostics methods, the opportunity has opened for the development of a new direction in perinatology - fetal surgery. Some pathological conditions of the fetus can be corrected before its birth, which prevents the birth of children in a serious condition. The first intrauterine surgical operation - replacement fetal blood transfusion - was performed in a severe form of hemolytic disease of the fetus by cordocentesis. However, the high frequency of intrauterine fetal death does not allow this method to be widely used.

Another area of fetal surgery is associated with puncture and emptying of pathological accumulations of fluid in the cavities of the fetus (hydrothorax, ascites, hydropericardium), which occur in cases of immune and non-immune fetal hydrops.

There have also been attempts at intrauterine treatment of fetuses with hydrocephalus, which consisted of implanting a ventriculoamniotic shunt to reduce intracranial pressure. Despite encouraging results from experimental studies, the value of clinical application of the method has not been definitively established: perinatal mortality among treated fetuses was 18%; 66% of survivors were found to have moderate to severe physical and mental developmental disabilities.

Promising are surgical interventions for reverse arterial perfusion in twins (a specific pathology in multiple pregnancies characterized by vascular communications between the fetuses, which can cause the death of one or the other twin). Reverse arterial perfusion occurs only in twins with fused placentas. In case of congestive heart failure (the appearance of pericardial effusion), a hydropericardial puncture is performed; in case of polyhydramnios, therapeutic amniocentesis. In addition, it is possible to perform ligation of communicating vessels in the umbilical cord or their laser coagulation, performed under endoscopic control.

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