Third trimester screening is a complex of studies conducted on a planned basis from 30 to 34 weeks of gestation and visualizing the fetus, its internal organs, placenta, umbilical cord, uterus and amniotic fluid. It allows to diagnose incorrect presentation of the unborn child, pathologies of organ development, umbilical cord entanglement of the fetal neck, fetoplacental insufficiency, the risk of premature birth. The 2D mode is used for the study. Screening is indicated for multiple and single pregnancies.
Third trimester screening does not require preparation. Transabdominal access is used in the study.
What shows third trimester screening
For third trimester screening, a two-dimensional mode is used. The thickness, localization and maturity characteristics of the placenta, the amount of amniotic fluid, the location and structure of the umbilical cord, the presentation and size of the fetus, the structure of its organs and bones of the skeleton are visualized. With an increased risk of congenital anomalies, ultrasound in 3D and 4D modes is additionally prescribed. If there are indications, dopplerometry is performed, reflecting the features of the blood flow of the uterus, placenta and fetus, as well as cardiotocography, revealing the nature of the fetal heartbeat. With the help of ultrasound screening in the third trimester, the following pathologies are diagnosed:
- The risk of premature birth. An important component of screening is the diagnosis of the risk of premature delivery. The high risk of this complication is indicated by an open internal or external pharynx, a shortened cervix, increased tone of the uterine walls.
- Entanglement of the umbilical cord. Ultrasound examination can reveal the situation of a single or multiple entanglement of the umbilical cord of the shoulder girdle, neck or abdomen of the fetus. For more accurate information about the location of the umbilical cord, a 3D ultrasound is prescribed, which is performed during screening and repeatedly before childbirth.
- Malformations of development. The most common malformations detected at the third ultrasound screening are the absence or incorrect structure of small parts of the body, for example, fingers, lips, jaw, palate. Pathologies of organs are also diagnosed – heart defects, disorders of the development of the spine, brain, lungs and genitourinary system.
- Pathological presentation of the fetus. In case of incorrect presentation, the fetus is located with the buttocks or legs down (pelvic presentation) or obliquely / transversely with respect to the longitudinal axis of the uterus. If the baby does not turn its head down by the time of delivery, presentation may require cesarean section surgery.
- Pathology of the placenta. The most dangerous pathology of the placenta includes its presentation – the location in the lower part of the uterus, partial or complete overlap of the internal pharynx, which complicates natural delivery. Placental hypoplasia is diagnosed at 32 weeks with a thickness of less than 23.5 mm, hyperplasia – with an index of more than 41.6 mm. Premature aging of the placenta is determined by its structure – the appearance of lobules earlier than 36 weeks.
Ultrasound is an affordable and safe method of research that can be carried out the required number of times at any stage of pregnancy. The procedure does not harm the unborn child and mother, has no side effects. As part of the standard screening, a two-dimensional ultrasound procedure is performed, which has a low cost in comparison with three-dimensional and four-dimensional studies, Dopplerometry. The limitation of this method is insufficient visualization in certain fetal positions, excess weight in the mother, hypertonicity of the uterus.