Third screening for pregnancy

, medical expert
Last reviewed: 19.10.2021

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The third screening for pregnancy (ultrasound examination of the fetus) - as well as the first two - is conducted to determine the conformity of fetal development with physiological standards.

Alas, no one is immune from violations of these norms, but to date, medicine has the ability to determine the presence of congenital abnormalities of the fetus, as well as to identify various deviations in the development of the unborn child that arise during pregnancy. This task is solved by prenatal (antenatal) diagnostics - biochemical and ultrasound screenings, which are carried out at different periods of pregnancy.

Biochemical screening is carried out in the first and second trimester - on the 11-13th and 16-18th obstetric weeks of gestation. Its purpose is to identify possible development in the fetus of certain genetically determined defects. Ultrasound screening for pregnant women is supposed to take place three times. The first time - at the 10-14th week, the second - between the 20th and 24th weeks.

The third screening during pregnancy (ultrasound examination of the fetus) is carried out at a period of 30-32 weeks.

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Terms of the third screening for pregnancy

The specified terms of biochemical and ultrasound screenings were chosen not accidentally, but are dictated by the fact that it is at these terms of bearing the child that the most important changes in his intrauterine development take place. So, the basic formation of the fetal organs systems by the 10-11th week is completed, and the pregnancy from the embryonic period enters the fetal, which lasts until the birth of the child.

Biochemical screening (blood test) of a pregnant woman is performed if she is at risk of having a child with Down syndrome, Edwards syndrome or neural tube defect (spina bifida, anencephaly, hydrocephalus). This group of doctors includes women who first became pregnant at the age of 35 and older, the presence of hereditary diseases among close relatives, previous births of sick children, as well as repeated spontaneous abortion (habitual miscarriages) in women. Biochemical screening is carried out by examining the blood level of human chorionic gonadotropin, alpha-fetoprotein and free estriol. The data of these analyzes with a sufficiently high degree of reliability make it possible to determine the risk of the appearance of the above pathologies in the child.

Ultrasound examination of structural malformations of the fetus is mainly based on the use of ultrasound in the second trimester of pregnancy. For example, the threat of Down syndrome is revealed by the thickness of the collar space in the fetus.

Women who are not at risk are ultrasound screenings that are performed three times during pregnancy. The exact terms of the third screening for pregnancy are related to the fact that during this period - at the 30-32th week - the growth and weight of the fetus increase, the head grows and the brain mass increases, the lungs develop intensively, the skin integrable and the subcutaneous adipose tissue. The volume of amniotic fluid in the uterus increases, and by 31-32 weeks the child should assume the position upside down - a physiologically normal presentation.

The norm for the third screening for pregnancy

To evaluate the future child's biometric data using ultrasound, special tables of the average physical and physiological parameters of the fetus were developed for all periods of pregnancy.

The norm for the third screening for pregnancy is:

  • length of the fetus (growth): 39.9 cm (30 weeks gestation), 41.1 cm (31 weeks), 42.3 cm (32 weeks);
  • weight: 1636 g (30 weeks of gestation), 1779 (31 weeks), 1930 (32 weeks);
  • biparietal size of the fetal head (head width according to the distance between the parietal tubercles): 78 mm (30 weeks gestation), 80 mm (31 weeks), 82 mm (32 weeks);
  • perimeter of the skull: 234 mm (30 weeks of gestation), 240 mm (31 weeks), 246 mm (32 weeks);
  • Chest diameter: 79 mm (30 weeks gestation), 81 mm (31 weeks), 83 mm (32 weeks);
  • abdominal girth: 89 mm (30 weeks gestation), 93 mm (31 weeks), 97 mm (32 weeks);
  • length of thigh: 59 mm (30 weeks), 61 mm (31 weeks), 63 mm (32 weeks).

The increase in the size of the abdomen of the fetus in comparison with its head and thorax against the background of thickening of the placenta refers to the clear signs of hemolytic disease of the newborn. This pathology occurs in the Rh rhesus-incompatibility of the blood of the mother and fetus and is expressed in the destruction of red blood cells in the blood of the child both before and after birth.

In addition, the experts refer to the excess of the average index on the girth of the stomach either to signs of fetal liver hypertrophy, or to signs of ascites - the accumulation of fluid in its abdominal cavity.

The length of the femur is also an important parameter of the third ultrasound screening during pregnancy. On it it is possible to judge the length of the limbs, and at a lower value of this indicator (in comparison with the norm and other biometric data), there are grounds to assume that the child has Nanism, that is dwarfish growth. This abnormality is associated with fetal pituitary dysfunction and growth hormone deficiency (somatotropin).

According to the statistics of the World Health Organization, up to 6% of children who are given birth every year by women from all over the world have certain congenital malformations. Existing preventive methods for determining the risk of the birth of a child with congenital pathology are screenings during pregnancy, including a third screening during pregnancy.

Indicators of the third screening during pregnancy

The parameters of the third screening in pregnancy - when examined by ultrasound - provide a basis for assessing the condition and degree of development of the fetus, its motor activity and position in the uterus (presentation), and draw conclusions about the placenta.

The third ultrasound screening in pregnancy is able to detect a violation of the placenta - fetoplacental insufficiency, which is a factor threatening the normal development of the fetus. A doctor who examines a pregnant woman at the end of the second or at the beginning of the third trimester can detect a disproportionate development of the fetus: a lag in body weight from the length, a mismatch between the size of the abdomen and the chest to the average standards (which indicates a delay in the development of the liver)

Also, during the third ultrasound screening, a special formula determines the amount of amniotic fluid. Their pathologically increased volume may be an indicator of intrauterine infection of the fetus or the presence of diabetes in the child.

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