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First Trimester Pregnancy Ultrasound examination

First Trimester Ultrasound examination: More information

First 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  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 easy for repetitive use.

The sonography is performed by a specialized medical imaging technologist, called a sonographer, an OB/GYN physician or a radiologist. The patient lies on their back or side with their lower abdomen uncovered. A gel is applied to the skin to transmit the ultrasound waves 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  first trimester pregnancy ultrasound is mostly used to estimate the delivery date (gestational age) or to confirm the presence of a live embryo (viability). In some cases it can be useful to detect early problems with the embryo or the pregnancy. The fisrt trimester begins at the first week of pregnancy and lasts throught the 15th week. However, it’s often impossible to view an embryo and to confirm pregnancy by ultrasound imaging before the 7th week. The sonographer sweeps the mother’s belly with the probe to investigate the size, shape and structure of the embryo 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 images are reconstructed in 2D, or even 3D, images on a screen as the sonographer and/or radiologist is carefully examining them.

The first trimester’s ultrasound can provide much information about the pregnancy by investigating the following:

Fœtus :

  • Cardiac activity : Heart beats per minute
  • Crown to rump length: Measurement from the tip of the head to the butt;
  • Biparietal diameter : Measurement of the head from side to side;
  • Measurement of the nuchal transluscency: if a prenatal screening for genetic defects was requested by the patient it is done only between 11 weeks + 4 days and 13 weeks + 6 days;
  • Nasal bone length.

In the case of a very young pregnancy where the structures cannot be visualised, the presence of a yolk sac with or without fetal pole will be assessed, and then measured. Depending on the gestational age, the presence, location, and number will be evaluated.

The head, limbs (legs and arms), heart beats, stomach and bladder will be checked in search of any potential anomaly.

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

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, calcifications(grade), hematomas, abruptios.

Sonography can also help 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 amniocenthesis or a chorionic villi sampling.

To proceed to an ultrasound during the first 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 first trimester ultrasound takes about thirty minutes. In the case of twins or multiples, this amount of time is multiplied by the number of embryos. 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.