Anatomical screening: what is examined
The second screening (18–21 weeks) is the most detailed examination of the fetal anatomy during the entire pregnancy. By this time, the organs have already been formed, and the size of the fetus and the volume of water allow them to be examined in detail. The doctor sequentially examines the brain with an assessment of the ventricles and cerebellum, facial structures and profile, a four-chamber section of the heart with output tracts, lungs, diaphragm, stomach, kidneys and bladder, anterior abdominal wall, spine along its entire length and limbs with segment counting.
At the same time, fetometry is performed: biparietal size, head and abdominal circumference, length of the femur. Based on these values, the estimated weight of the fetus and its compliance with the term are calculated. The location of the placenta and its distance to the internal os, the structure and degree of maturity, as well as the amniotic fluid index are assessed. The length of the cervix is measured separately - its shortening is a leading predictor of premature birth and requires timely intervention.
Third trimester screening (30–34 weeks) shifts the focus from anatomy to function: the main factors are the rate of fetal growth, its position and presentation, the condition of the placenta and blood flow. The main task is to timely identify growth retardation and placental insufficiency, which determine the tactics of labor management.
Why are planned studies needed?
Ultrasound is the main tool for monitoring pregnancy. It confirms that the fetus is alive and developing on time, shows the number of fetuses, the position of the placenta, the volume of amniotic fluid and the condition of the cervix. Most serious malformations are detected by ultrasound, and at a time when it is still possible to make an informed decision.
There are usually three planned examinations during pregnancy - one in each trimester, in strictly defined windows. Outside of these periods, some measurements lose diagnostic value: for example, the thickness of the collar space can be correctly assessed only from the 11th to the 13th week plus 6 days.
Safety of ultrasound during pregnancy
Ultrasound is not ionizing radiation: it does not accumulate in tissues and does not have a radiation dose, unlike X-rays and computed tomography. Over decades of use, no data have been obtained on the harmful effects of diagnostic ultrasound on fetal development.
In this case, the principle of reasonable sufficiency applies: the study is performed according to indications and in the optimal time frame, and the power and exposure time are kept at the minimum required level. This applies separately to the Doppler and 3D/4D modes - they are used consciously, and not “just in case” at each visit.
What affects image quality
The clarity of the picture depends on the age, position of the fetus, the thickness of the anterior abdominal wall, the amount of amniotic fluid and the presence of a scar on the uterus. If the fetus turns its back to the sensor or covers its face with its hands, some structures cannot be examined.
In such cases, the doctor may ask you to walk, change your body position, or come back in a few days. This is not a sign of pathology, but a common technical situation, especially common during 3D/4D examination.