What is measured at 11–13 weeks
The first screening is carried out in a narrow window - from 11 weeks to 13 weeks and 6 days, when the coccygeal-parietal size of the fetus is from 45 to 84 mm. Before this period, the structures are still too small; later, the collar space spontaneously decreases, and the main marker loses its meaning.
The key indicator is the nuchal translucency thickness (NTT): the accumulation of fluid under the skin of the fetal neck. Its increase is associated with an increased risk of Down syndrome and other chromosomal abnormalities, as well as heart defects. Additionally, the presence and length of the nasal bone, blood flow in the venous duct and through the tricuspid valve, heart rate, as well as anatomy in the accessible volume for the period are assessed: limbs, abdominal wall, bladder, cerebral hemispheres.
It is important to understand: screening does not make a diagnosis. It calculates individual risk by combining ultrasound data with maternal age and blood biochemical markers (β-hCG and PAPP-A). A high calculated risk is not a death sentence, but an indication for clarifying methods: a non-invasive prenatal test using maternal blood or invasive diagnostics with karyotype testing.
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.