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Third Trimester Pregnancy Ultrasound examination

Third Trimester Ultrasound examination: More information

Third trimester pregnancy ultrasound examination, or Sonography, is a medical imaging technique that allows viewing inside the body. It is done using a transducer, also called a ‘probe’, which emits high frequency sound waves (ultrasounds). When reflected by the body’s internal structures, these waves produce images.

Since ultrasounds are stopped by gas and by hard structures such as bone and calcification, some internal organs are not well viewed with sonography which, therefore, is not the primary choice of exam. It is the case for the lungs, bowels, stomac, and brain. Unlike MRI, CT scan and X-ray, no ionizing radiations are used during sonography. It is painless and non-invasive, which makes it safe for repetitive use.

The sonography is performed by a specialized medical imaging technologist, called a sonographer, an OB/GYN or a radiologist. The patient lies on their back or side with their lower abdomen uncovered. A transmission gel is applied to the skin and ease the movements of the probe. This type of examination is usualy painless, but the patient may experience some discomfort as the sonographer presses with the probe.

The  third trimester obstetrical ultrasound is mostly used to assess adequate fetal growth and its biophysical profile. It is generally performed between the 28th and 32nd week of gestation, however, it can be done before or after that period depending on the doctors or midwife’s recommendation to check on some aspect of the pregnancy or to follow-up on previously diagnosed anomalies. The sonographer sweeps the mother’s abdomen with the probe to investigate the size, shape and structure of the foetuses organs and limbs, as well as the mother’s uterine health.  If necessary, the technologist may perfom a trans-vaginal examination using a specialized probe, allowing more detailled images. In this case, the patient will, after emptying their bladder, remove clothing for their bottom and adopt a gyneacological position. The waves are reconstructed into 2D, or even 3D, images on a screen as the sonographer and/or radiologist is carefully examining them.

The third trimester obstetrical ultrasound can provide much information about the pregnancy by investigating the following:

Fœtus :

  • Biparietal diameter : Measurement of the head from side to side;
  • Distance occipito-frontale: Distance between the occiput (back of the head) to the forehead;
  • Head circumference;
  • Abdominal circumference;
  • The lenght of the femur;
  • Cardiac activity: beats per minute
  • Ombilical cord Doppler;
  • Cerebral artery Doppler;
  • Measurement of the cardiac septum;
  • Estimation of the baby’s weight.

Furthermore, the presence, location, shape and number (when applicable) of the spine, stomach, kidneys, bladder, limbs, heart chambers,outflow tract  and the aortic arch, as well as facial features like the nose, lips and profile will be investigated in search of any potential anomaly.

If there is a multiple pregnancy (twins, triplets, or more), this detailed investigation will be repeated for each feotus.

Mother :

  • Uterus and cervix: fibroids, cysts, masses, cervix length and early dilatation.
  • Doppler of the uterine artery;
  • Ovaries: Size, presence of cysts or masses;
  • Placenta: Size, shape, location, distance from cervix, grade, calcifications, hematomas, abruptios.
  • Amniotic fluid : Quantity (volume, amniotic index ), presence of sludge;
  • Membrane : Presence of synechiae or smniotic membrane.

Sonography can also help to show abnormal lymph nodes (lymphadenopathy), suspicious masses and the collection of free fluids (ascitis) or blood.

In some cases, ultrasounds are useful to guide interventions allowing more elaborate testing during pregnancy, like an amniocentesis

Contrary to the first and second trimester ultrasound where the patient’s bladder must be full, no preparation is needed for the third trimester’s ultrasound.

The third trimester ultrasound takes about thirdy minutes. A detailed report is written by the radiologist or OB/GYN  and is then transmitted to the patient’s physician or midwife who will make decisions regarding prenatal follow-ups and, if necessary, further examinations.