Frequently referred to as the 20 week fetal anomaly screen, this Ultrasound is typically performed between 18-20 weeks gestation. It is preferred that it be done at 19-20 weeks as the organs are just that much larger in size and easier to evaluate than at 18 weeks. The ultrasound is standard for assessing fetal growth, viability, placental location and an in depth view of fetal anatomy. It must be stressed that not all fetal anatomy is seen at this or any other sonogram. At 20 weeks gestational age, although all fetal organs are present, some may not be fully functional yet (i.e. lung maturity) and many cannot be seen due to the sheer size of the organs. For example, the size of the fetal heart is about the size of a grape at this time, yet a 4 chamber view of the heart and cardiac outflow tracts can often be seen normally. The fetal calvarium, including brain anatomy may be seen normally. Some fetal anomalies are detected at this stage due to the fact that there are no “barriers” to the sound waves and their reflections. Calcification (fetal, maternal bone) and air (maternal bowel) can obscure visualization of fetal anatomy at any time during the pregnancy. Twenty weeks gestation was chosen as the ideal time to view anatomy because despite the small size of the organs and the fetus itself (a fetus will weigh one pound between 21-22 weeks gestation) the bone calcification is minimal. This allows the sound waves to easily pass through the fetus and send back reflective images to the screen.
In the 3rd trimester of pregnancy, the organs are larger but bone structure obscures visualization. And, in truth, some fetal anomalies are not static but developmental. A fetal cardiac 4 chamber view may be normal at 20 weeks but as the heart grows and continues to develop, certain abnormalities may develop. A normal aortic valve at 20 weeks may become strictured at 37 weeks. The placenta may be normal at 20 weeks but prematurely age after 30 weeks due to gestational diabetes or pregnancy induced hypertension. In other words, we see a lot but we cannot see it all.
So, what can and do we look at on a 20 week sonogram? First, we like to know any and all family genetic history before we get started. This is so we may target in on areas that may need a more attentive eye to determine normal vs. abnormal anatomy. Second, we never need a full maternal bladder, so don’t rush to overhydrate before the exam.
The 20 week sonogram looks to measure and/or view the following fetal/maternal anatomy:
*maternal cervical length
*maternal ovaries and adnexae
*fetal position (vertex, breech, transverse)
*placental position (anterior, posterior, fundal, previa), umbilical cord insertion into both the placenta and fetal abdomen
*fetal brain: lateral ventricles, cerebellum, cisterna magna
*fetal heart: 4 chamber, 2 atria, 2 ventricles, left and right outflow tracts, aortic arch, cardiac rhythm, fetal heart rate
*fetal spine: cervical, thoracic, lumbar and sacrum
*fetal face: lens of the eye, nasal bone, lips
*umbilical cord: 3 vessel cord, 2 arteries, 1 vein
*amount of amniotic fluid (AFI=amniotic fluid index)
*fetal stomach, kidneys, arms, legs, digits in hands and feet, fetal diaphragm
Many of the measurements of the fetal head, abdomen and femur length will verify the fetal gestational age and approximate weight. At this point we cannot determine fetal length as they are all curled in the fetal position. I would love to reach in and stretch them out to measure and view anatomy…but I simply cannot do it!
In sum, the 20 week sonogram is scheduled for an optimal time to view anatomy even with size limitations.