Ultrasound examination in the 2nd trimester is typically performed between 21 to 24 weeks of gestation, for the main purpose of assessing in detail the anatomy of the fetus. Also, an assessment is made regarding the amount of amniotic fluid, the position of the placenta, the degree of placental maturity and of the cervical length.
Finally, an evaluation of the blood flow of the uterine arteries may also be performed, as an abnormal blood flow is associated with the risk of pre-eclampsia or intrauterine growth retardation.
During this ultrasound examination, the detailed investigation of the anatomy of the fetus includes:
- structure of the heart and chest
- the head/skull and the brain
- the spine and neck
- the face
- urinary tract and kidneys
- gastrointestinal tract and abdominal wall
- the upper and lower limbs
- the sex of the fetus
Abnormalities involving these ultrasound markers may indicate the presence of a chromosomal abnormality in the fetus and are assessed together with the results of the 1st trimester ultrasound examination. This assessment is not absolute and may indicate an increased risk for chromosomal abnormalities, leading to a recommendation for amniocentesis and genetic testing of the fetus through classical or molecular karyotype.
It is important to note that this ultrasound examination is not in a position to reveal all possible structural malformations of the fetus. Therefore, a normal result serves to minimize considerably the associated risks, but cannot totally exclude the presence of an abnormality in the fetus due to inherent limitations of the procedure.