20 Weeks Ultrasound Examination

The 20 weeks ultrasound examination ("detail ultrasound") can be done from 19 to 22 weeks.  

The scan is usually performed transabdominally. Sometimes, it may be necessary to do the scan transvaginally. 

The aims of 20 weeks ultrasound examination are to:

  • Evaluate the structural development of the baby, with special attention  to the brain, spine , face, heart, stomach, bowel, kidneys and limbs. If you wish, the fetal gender can be determined.
  • Determine the growth of the baby,
  • Determine the position of the placenta and the amount of amniotic fluid,
  • Measure the length of the cervix if needed to assess the risk of preterm labour (especially in the first pregnancy, or in case of a previous preterm delivery) and
  • Measure the blood flow in the uterine arteries to assess the risk of growth restriction and pre-eclampsia.

Some specialized forms of the 20 weeks ultrasound include fetal echocardiography, where baby’s heart is examined in more detail, and fetal neurosonography, where the baby’s brain is examined in more detail. This is usually done if a problem is detected, or where there is a high risk of a specific abnormality in the heart or brain respectively.

Most babies (>95%) are born healthy. Of  babies with serious structural abnormalities, the 20 weeks ultrasound scan can detect about half. Whether it is detected or not, can depend on a number of factors, such as:

  • The specific condition:  some conditions, such as spina bifida, anencephaly (deficient development of the skull) and gastroschisis (a defect in the abdominal wall), are detected in more than 90% of cases. Some conditions (such as cerebral palsy or autism) are usually NOT seen on ultrasound. Some conditions (such as certain forms of dwarf growth or hydrocephalus) only develop later on in pregnancy.
  • How well the baby is seen on ultrasound.  This in turn depends on the quality of the ultrasound equipment and the skill of the person doing the ultrasound, but also on factors like the pregnancy duration, the position of the baby and placenta, the amount of amniotic fluid and the amount of fluid or fat or possible scar tissue  in your abdominal wall. Especially if your BMI is above 30, it can be tricky to visualize every detail well.

The 20 weeks ultrasound is not the ideal screening test for Down’s syndrome, as it is less accurate than the evaluation at 12 weeks. If you have missed the 12 weeks’ evaluation and you do want to know what the risk of Down syndrome is, the options would include a blood test done between 15 and 19 weeks (the “triple test”) alone or in combination with the 20 weeks ultrasound, cell free fetal DNA testing (NIPT) or an invasive test.

If something out of the ordinary is seen, further tests might be recommended. These tests might include blood tests or an invasive test such as an amniocentesis, consultation with other specialists (such as geneticists), or repeating the ultrasound examination after some time.

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