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Third Trimester screening Tests: Preparation and procedures

Third Trimester screening Tests


Screening is a comprehensive examination carried out at certain times of pregnancy. It aims to identify severe fetal pathology and establish additional risks for the baby and childbirth. It is necessary to undergo three screening tests for the entire period of bearing a child. 

The third screening during pregnancy is performed from 32 to 34 weeks. It is carried out using ultrasound and sometimes CTG and determines the correspondence of the size of the fetus to the gestational age and the search for abnormalities that appear later than the others.

Third trimester screening tests

The study aims to detect severe malformations and chromosomal abnormalities in primary and secondary screening.

The screening tests conducted during the third trimester assess the size of the fetus compared to the gestational age and detect any anomalies which may have developed earlier than expected.

It is possible to determine the week of pregnancy by the last screening, but it should be considered that the growth rate of the fetus may depend on several factors:

  • The state of the fetoplacental system.
  • The nature of the mother’s diet.
  • Extragenital pathology.
  • The presence of intrauterine infection.
  • Bad habits and exposure to toxic substances.

The timing is set so that the pathology of the fetus can be corrected until it’s maturity and choose the appropriate delivery method.

An obligatory method of third trimester screening test is an ultrasound of the fetus. It requires a modern device that allows you to conduct fetal dopplerometry simultaneously. In some cases, an extended study is carried out for the determination of the biophysical profile of the fetus which includes ultrasound and CTG according to a certain technique.

Biochemical analysis in the 3rd trimester to determine alpha-fetal protein and hCG is not performed. For this study, a strict deadline is set 16-18 weeks. Later, it loses its information scope.

Third Trimester screening Tests: Ultrasound

Special preparation for ultrasound is not required. The fetus is already large so the doctor will see each body part separately. The study is necessary to clarify the anatomical and functional features of the fetus if there are variations from the norm. In most cases, they are diagnosed during the 1st and 2nd screenings.

The following indicators are used to measure if the size of the fetus is consistent with the gestational age:

CriteriaNormal Size
Biparietal size75-89 mm
Abdominal circumference258-314 mm
Thigh length56-66 mm
Fronto-occipital size95-113 mm
Head circumference283-325 mm
Humerus52-62 mm
Forearm45-53 mm
Lower leg52-60 mm
Normal size of the fetus body for third trimester screening test

If the size of the fetus is smaller than what is typically seen, it is diagnosed as Intrauterine growth restriction.

The anatomical features of the organs are evaluated in detail. Medical search is aimed at identifying late defects. Particular attention is paid to the nervous system. During the process, issues such as a vein of Galen aneurysm, hydrocephalus, subarachnoid cysts, or bleeding spots can be identified.

During the last three months of gestation, it is possible to diagnose congenital tumors such as the following: 

  • Teratoma
  • Lymphangiomas
  • Hemangiomas
  • Cysts and intracranial structures.

The state of the digestive tract is assessed. During the ultrasound, obstruction of the large and small intestines can be detected. When examining the musculoskeletal system, various variants of dysplasia are noticeable.

Fetal life support systems are studied. When examining the placenta, the degree of its maturity is established. For a period of 32-34 weeks, the normal indicator corresponds to the first degree, but as the 35th week approaches, signs of the 2nd degree of maturity are allowed.

Third Trimester screening Tests: Ultrasound
Third Trimester screening Tests: Ultrasound

The doctor evaluates the heart’s working and the umbilical cord’s condition. At this time, signs of Umbilical cord entanglement also known as Nuchal cord may appear, but they do not always persist by the time of delivery.This is not a threat if there are no signs of fetal hypoxia.

It is important to assess the condition and volume of amniotic fluid, which can change up or down. Polyhydramnios is a frequent companion of infectious diseases, and oligohydramnios is an anomaly in the development of the fetus.

The functional state of the fetus reflects the uteroplacental blood flow. It is examined during dopplerography. Percentile curves determine the vascular resistance index for a given gestational age.

During a third trimester ultrasound, what would a doctor expect to observe in a fetus?

The biophysical profile of the fetus is a diagnostic method that considers CTG data and ultrasound indicators. A specific preparation is not required. It is enough to give the woman time to calm her breathing and heartbeat after the trip to the clinic.

The procedure is difficult because of its duration. To fully assess the fetus’s condition, conducting an ultrasound scan for 30-40 minutes is necessary. During the study, pay attention to the following indicators:

  • Respiratory movements.
  • Physical activity.
  • Fetal muscle tone.
  • The volume of amniotic fluid.
  • Maturity of the placenta.

For each criterion, ratings range from 0 (low/absent) to 2 (normal).

During CTG, a non-stress test is performed. The readings of the baby’s normal values indicate that within 20 minutes, there is an occurrence of no less than 5 episodes of an elevation in heart rate of 15 beats per minute for a period of 15 seconds.

Motor activity is assessed for 30 minutes. A healthy fetus shows 3 or more generalized movements. Whilst in the womb, babies cannot take full breaths, but they can still perform actions that appear like breathing. Generally, each respiratory cycle lasts 60 seconds and occurs every half an hour.

Muscle tone in the fetus cannot be checked. The doctor assesses the child’s ability to flex their arm or leg from an extended position to a bent posture during the entire duration of the ultrasound examination, which should occur at least once.

According to the study results, the number of points is calculated. Normal indicators – 8-12 points. If the fetus gains 6-7, this indicates a dubious condition and requires careful monitoring and conservative treatment. A sum of 5 points or less indicates hypoxia and intrauterine suffering then there is a high risk of complications.

Screening examination during pregnancy is one of the required diagnostic methods. It is carried out in antenatal clinics equipped with a high-quality ultrasound machine. If deviations are detected, the woman is sent for additional examination to a higher-level medical care institution to clarify the diagnosis.