20 weeks ultrasound
The 20 weeks ultrasound examination 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 and
- Measure the blood flow in the uterine arteries if needed to assess the risk of growth restriction.
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:
- 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.
- The 20 weeks ultrasound is not a good screening test for Down’s syndrome. Because about half of babies with Down’s syndrome would have some major or minor abnormality which is visible on the 20 weeks ultrasound, it is better to do a 12 weeks ultrasound if you want to know what your baby’s risk of Down’s syndrome is.
If something out of the ordinary is seen, we will try to make as accurate a diagnosis as possible. This might entail further tests such as blood tests or an amniocentesis, consultation with other specialists (such as geneticists), or repeating the ultrasound examination after some time.