Second Trimester Pregnancy Ultrasound Examination
Second Trimester Ultrasound examination: More information
Second trimester 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. Sonography can also be useful to guide the punction of an organ, an intra-articualr injection or a biopsy.
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 for examining them. It is the case for the lungs, intestines, 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 imagery technologist, called a sonographer, an OB/GYN or a radiologist. The patient lies on their back or side with their lower abdomen uncovered. A gel is applied to the skin to help with the ultrasounds’ transmission 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 pregnancy’s second trimester ultrasound is mostly used to perform a study of the feotus’ morphologic features. Generally, it’s done between week18 and week 22, ideally at week 19. It is a complex examination during which the sonographer has to assess all of the baby’s body parts in order to verify that her development and well being are adequate. The sonographer sweeps the mother’s belly with the probe to investigate the size, shape and structure of the feotus’ 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 any 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.
Contrary to popular belief, the second trimester ultrsound study is not meant to find out the baby’s gender! Although it is generally possible to view the gender at this point, the examination is meant to keep the feotus and pregnancy’s evolution in check by evaluating the following:
- Crown to rump length: Measurement from the tip of the head to the tip on the butt;
- Biparietal diameter : Measurement from the side of the head to the other;
- Distance occipito-frontale: Distance between the occiput (back of the head) to the forehead;
- Head circumference;
- Abdominal circumference;
- The lenght of the femur, humerus, radius and tibia;
- The size of the cerebellum;
- Measurement of the posterior fossa;
- Measurement of the nuchal fold;
- Measurement of the occipital horn;
- Estimation of the baby’s weight.
Furthermore, the presence, location, shape and number (when applicable) of the cerebral ventricles, spine, stomach, kidneys, bladder, limbs, fingers and toes, heart chambers, ombilical cord and it’s insertion as well as facial features like the nose, lips and orbits will be investigated in search of any potential anomaly. The heart’s structure will be assessed (heartbeat and arteries locations). Using a Doppler, the sonographer will also ensure the blood is properly flowing within the kidney and ombilical arteries.
If there is a multiple pregnancy (twins, triplets, or more), this detailed investigation will be repeted for each feotus.
- 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, abruptions.
- Amniotic fluid : Quantity, presence of sludge;
- Membrane : Presence of synechiae.
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 amniosynthesis or a chorionic sampling.
To proceed to an ultrasound during the second trimester of pregnancy, the patient must have a full bladder. They must drink one liter of clear liquid one hour prior to the examination. Then, the patient must not urinate to leave time for the bladder to fill up.
The morphologic study takes about forty 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 check-ups and, if necessary, further examinations.