5 “myths” and realities concerning the specialized 1st trimester pregnancy ultrasound

The specialized 1st trimester pregnancy ultrasound has now been established as a necessary test for future mothers. This test, which is performed from the 11th to the 13th week +6 days of pregnancy or when the embryo is 45 to 84mm, is said to be the test in which the probability of the presence of a Down syndrome fetus is calculated. But let’s talk about some misconceptions (“myths”) about this test.

1st “myth”: this ultrasound is only done to diagnose or exclude the presence of Down syndrome in the fetus.

Reality: NEVER can any syndrome be diagnosed or excluded only by the ultrasound image of the fetus. The external appearance of the fetus as well as the characteristics tested can only identify those fetuses that are more likely to have some chromosomal abnormality. Diagnosis requires chromosome check of the fetus (which requires invasive prenatal testing)

Reality: it is equally important during the 1st trimester check to make a correct assessment of the fetus’ age based on its size because it is the pregnancy stage where the size of the fetus is unaffected by the characteristics of its parents. 

Reality: it is necessary to check the fetus’s anatomy even at this early stage of pregnancy.

2nd “myth”: the fetus is too small in size so no anatomical abnormalities can be detected

Reality: Although the fetal size is relatively small (45-84mm during the 1st trimester check), a first estimation of the embryo’s anatomy can be made. For example:

  • From the head severe brain abnormalities can be detected or suspected
  • From the face large gaps can be found in the palate (cleft palates), the lenses in the eyes can be checked and any micrognathia can be shown.
  • Severe heart abnormalities can be diagnosed (for example a heart in an abnormal position, abnormal position of the aortic and pulmonary artery, large gaps in the diaphragm separating the 2 ventricles of the heart etc.)
  • Early diagnosis of diaphragmatic hernia, which means presence of abdominal organs in the chest. 
  • Check of the fetus’ limbs (even counting the fingers if the examiner has a high-resolution ultrasound machine)
  • Check the presence of the kidneys and / or the suspicion of agenesis.
  • There may still be suspicion of spinal bifida (= a severe condition which affects the spine and the skin above it)

3rd “myth”: The absence of nasal bone implies that the fetus has Down syndrome. In reality, in 60-70% of fetuses with trisomy 21 and about 2% of chromosomal normal fetuses, the nasal bone is not visible during ultrasound examination at 11-13 + 6 weeks.

4th “myth”: The presence of an increased amount of fluid in the fetus’ neck (= nuchal translucency) implies that the fetus has Down syndrome.

Reality: Fetal nuchal translucency increases with each month of pregnancy. Therefore, to consider that nuchal translucency is increased, consideration should be given to the size of the embryo. For example, a 2.5mm cervical translucency when the fetus has a length of 55mm is considered increased, while when it is 70mm it is within normal limits. 

Reality: Increased nuchal translucency is not only associated with chromosomal or genetic abnormalities (e.g., Down syndrome). It can even be observed in normal fetuses or fetuses with cardiac, skeletal or structural abnormalities

5th “myth”: the calculation of the possibility of presence or not of chromosomal abnormalities is made only by the mother’s age, the presence or absence of the nasal bone, the size of the nuchal translucency, the fetal heart rate and the hormones secreted by the placenta of the fetus .

Reality: In order to reduce the false positive results and to improve the diagnosis of Down syndrome fetuses, a check is being done of the flow into the ductus venosus (a vessel carrying oxygenated blood to the fetus’ brain) and through the tricuspid valve.

The developments concerning the ultrasound of the 1st trimester of pregnancy are rapid and it is of vital importance to monitor and implement them in the daily practice of the new guidelines for this examination. This of course requires training and acquirement of these practices in appropriate centers rather than just experience!