Determination of alpha-fetoprotein
The mother's serum is tested for alpha-fetoprotein at the 17th week of pregnancy. Its level is increased if the fetus has an open (but not closed) neural tube defect or an umbilical hernia or when there are vices such as the urethral posterior valves and the Shereshevsky-Turner syndrome in twin pregnancy. The level of alpha-fetoprotein is reduced in pregnancy with fetus with Down syndrome and in mothers suffering from diabetes mellitus. Because this test is non-specific, it is used for pre-screening: those with positive testing may be offered a further examination.
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Triple blood test
Determination of the level of alpha-fetoprotein, unconjugated estradiol and human chorionic gonadotropin in the blood serum of the mother is carried out, correlating with the age, the mother's body weight and the gestation period. This makes it possible to assess the risk of a fetus having a neural tube defect and Down's syndrome. If this test and subsequent screening of mothers at risk were available to all mothers, then, as shown in a broad prospective study (N = 12,603), an early recognition of Down syndrome would increase from 15 to 50% (Scheurmier hypothesis).
Amniocentesis is performed under ultrasound at the time of pregnancy about 18 weeks. The frequency of fetal loss after such manipulation is about 0.5%. In the amniotic fluid, the alpha-fetoprotein content is determined (this is a more accurate study for the detection of neural tube defects than the determination of alpha-fetoprotein in the mother's blood serum), and amniotic fluid cells are cultured to study the karyotype, to determine the activity of the enzymes of the gene analysis. The study of cell culture takes 3 weeks, so abnormal pregnancy can be interrupted somewhat later.
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Chorionic villus biopsy
On the 8-10th week, samples of the developing placenta are taken for examination with a transcervical catheter or transabdominal needle under ultrasound control. Karyotyping takes 2 days, to determine the activity of enzymes and gene analysis requires 3 weeks, so the possibility of interrupting abnormal pregnancy is provided earlier, with greater safety and less stressful stresses than after amniocentesis. The frequency of fetal loss after a chorionic villus sampling is about 3% (0.8% higher than after amniocentesis). This procedure does not allow to reveal defects of the neural tube of the fetus,
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Ultrasound with high resolution
Starting from the period of 18 weeks, an experienced specialist can detect an increasing number of external and internal structural anomalies.
It is performed starting from the 18th week under ultrasound control. External fetal malformations can be detected, fetal blood samples and biopsies of its organs taken. The frequency of fetal loss is about 4%.
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