Third screening in pregnancy

The third screening during pregnancy (ultrasound examination of the fetus) – like the first two – is carried out to determine whether the development of the fetus meets physiological norms.

Unfortunately, no one is immune from violations of these norms, but today medicine has the ability to determine the presence of congenital pathologies of the fetus, as well as to identify various developmental deviations of the future child that arise during pregnancy. This problem is solved by prenatal (prenatal) diagnostics - biochemical and ultrasound screenings, which are carried out at different stages of pregnancy.

Biochemical screening is performed in the first and second trimesters - at 11-13 and 16-18 obstetric weeks of gestation. Its purpose is to identify the possible development of certain genetic defects in the fetus. Pregnant women are required to undergo ultrasound screening three times. The first time - at 10-14 weeks, the second - between 20 and 24 weeks.

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

Timing of the third screening during pregnancy

The specified periods of biochemical and ultrasound screenings were not chosen by chance, but are dictated by the fact that it is at these stages of gestation that the most important changes in the intrauterine development of the child occur. Thus, the basic formation of the fetus's organ systems is completed by the 10th-11th week, and the pregnancy enters the fetal period from the embryonic period, which lasts until the birth of the child.

Biochemical screening (blood test) is performed on a pregnant woman if she is at risk of giving birth to a child with Down syndrome, Edwards syndrome or neural tube defect (spina bifida, anencephaly, hydrocephalus). Doctors include in this group women who became pregnant for the first time 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 termination of pregnancy (habitual miscarriages) in a woman. Biochemical screening is performed by testing the blood for the level of human chorionic gonadotropin, alpha-fetoprotein and free estriol. The data from these tests with a fairly high degree of reliability allow us to determine the risk of the above-mentioned pathologies in a child.

Ultrasound examination of structural fetal malformations is mainly based on the use of ultrasound in the second trimester of pregnancy. For example, the threat of Down syndrome is detected by the thickness of the nuchal translucency in the fetus.

Women who are not at risk undergo ultrasound screenings three times during pregnancy. The specific timing of the third screening during pregnancy is related to the fact that during this period - at 30-32 weeks - the growth and weight of the fetus increases significantly, its head grows actively and the mass of the brain increases, the lungs develop intensively, the skin thickens and subcutaneous fatty tissue is formed. The volume of amniotic fluid in the uterus increases, and by 31-32 weeks the child should assume a head-down position - a physiologically normal presentation.

Norm for the third screening during pregnancy

To assess the biometric data of the unborn child using ultrasound, special tables of average physical and physiological parameters of the fetus at all stages of pregnancy were developed.

The norm for the third screening during pregnancy is:

• fetal length (height): 39.9 cm (30 weeks of pregnancy), 41.1 cm (31 weeks), 42.3 cm (32 weeks);
• weight: 1636 g (30 weeks of pregnancy), 1779 g (31 weeks), 1930 g (32 weeks);
• biparietal diameter of the fetal head (width of the head by the distance between the parietal tubercles): 78 mm (30 weeks of pregnancy), 80 mm (31 weeks), 82 mm (32 weeks);
• skull perimeter: 234 mm (30 weeks of pregnancy), 240 mm (31 weeks), 246 mm (32 weeks);
• chest diameter: 79 mm (30 weeks of pregnancy), 81 mm (31 weeks), 83 mm (32 weeks);
• abdominal circumference: 89 mm (30 weeks of pregnancy), 93 mm (31 weeks), 97 mm (32 weeks);
• femur length: 59 mm (30 weeks), 61 mm (31 weeks), 63 mm (32 weeks).

An increase in the size of the fetus's abdomen compared to its head and chest against the background of thickening of the placenta is one of the clear signs of hemolytic disease of the newborn. This pathology occurs when the blood of the mother and fetus is Rh-incompatible and is expressed in the destruction of red blood cells in the child's blood both before and after birth.

In addition, experts attribute abdominal circumference exceeding the average statistical value either to signs of fetal liver hypertrophy or to signs of ascites – accumulation of fluid in the abdominal cavity.

The length of the femur is also an important parameter of the third ultrasound screening during pregnancy. It can be used to judge the length of the limbs, and if this indicator is lower (compared to the norm and other biometric data), there is reason to suspect that the child has nanism, that is, dwarfism. This anomaly is associated with dysfunction of the fetal pituitary gland and deficiency of growth hormone (somatotropin).

According to statistics from the World Health Organization, up to 6% of children born annually by women around the world have some kind of congenital defect. Existing preventive methods for determining the risk of having a child with a congenital pathology are screening during pregnancy, including the third screening during pregnancy.

Indicators of the third screening during pregnancy

The results of the third screening during pregnancy - during examination using ultrasound - provide the basis for assessing the condition and degree of development of the fetus, its motor activity and position in the uterus (presentation), as well as drawing conclusions about the condition of the placenta.

The third ultrasound screening during pregnancy can reveal a placental dysfunction - fetoplacental insufficiency, which is a factor that threatens the normal development of the fetus. A doctor examining a pregnant woman at the end of the second or beginning of the third trimester can reveal disproportionate development of the fetus: body weight lagging behind length, discrepancy between the size of the abdomen and chest and average norms (which indicates delayed liver development), etc.

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