A Sonographer’s Detail of the Level 1 Anatomy Screen
By Sandra Minck, RDMS
A brief insight into an obstetrics ultrasound scan – for those who have expertize in other areas of ultrasound.
Better known to many patients as “the scan where one can tell gender,” we sonographers already know, even if it isn’t your specialty, that determining fetal sex is but a side perk of this important diagnostic screen. Much like the art of pastry to the culinary chef, the echocardiographer may find himself or herself lost in the amnion universe with an ever-moving target. This post is dedicated to the Level 1 anatomy screen in general obstetrics and the detail we seek to document aimed at those sonographers who aren’t trained in obstetrics but would like to know just a little bit more!
When Is the Anatomy Screen Performed?
Most healthcare providers order this screen between approximately 18-21 Weeks of pregnancy. By the mid-second trimester, fetal size, brain, and organ development are typically adequate for optimal evaluation.
The Purpose of the Anatomy Screen
Diagnosing serious structural anatomical malformations early enough to ensure the healthiest pregnancy and delivery is the goal of the anatomy screen examination. In the event of such a finding, physicians can then begin preparation for the management of care of mother and baby.
This plan typically includes parent/family education and counseling and Maternal Fetal Medicine consultations. Additional coordination may be required for delivery at a facility with specific NICU accommodations for a neonate’s immediate surgical intervention at birth.
What We Document in the Anatomy Screen
Even in light of normal 1st Trimester genetic testing, structural malformations can go undetected without the crucial component of visual observation. Let’s diver deeper into this documented detail.
The American College of Radiology (ACR), the American College of Obstetricians and Gynecologists (ACOG), and the American Institute of Ultrasound in Medicine (AIUM) concur for practice guidelines for obstetrical ultrasound. This list is meant to be utilized by practitioners as an educational tool and not an inflexible rule. A departure in itself does not imply substandard care.
Standard imaging protocol…
In no particular order is recommended as follows:
- LMP entered for establishment of gestational age
- Fetal number and presentation — obviously, multiple gestations require more detailed documentation with respect to chorionicity, amnionicity, biometry and amniotic fluid comparison, placental number/ location, and fetal genitalia (if possible)
- Fetal cardiac activity documented with M-mode, reporting abnormal heart rate or rhythm; basic size and position of the heart; four chambers; outflow tracts, if possible; and any structural malformations suspected
- A qualitative assessment of amniotic fluid volume for the experienced sonographer or the AP diameter of the deepest single pocket
- Placental grade, location, and distance to the internal cervical os
- Placental cord insertion and number of vessels within the cord, noting any discrepancy in arterial AP dimension
- Cervical length to rule out a shortened or incompetent cervix — care should be taken to measure the cervix with an empty patient bladder ensuring no presence of a contraction which can create an artificially-lengthened Transvaginal sonography is recommended when suspicious for shortening.
- A biometry or fetal growth assessment including the BPD or biparietal diameter and HC or head circumference, AC or abdominal circumference, and FL or femur length all shown below:
Figure 1. BPD/HC
The fetal head is measured at the level of the thalami and cavum septum pellucidum. The cerebellum, choroid plexus, cisterna magna, midline falx are also imaged. Unusual head shape noted. Intracranial ventricles should measure less than 1 cm.
Figure 2. AC
Figure 3. FL.
These measurements are compared to gestational age for consistency or discrepancy in an effort to determine a change in EDD (estimated due date) versus possible growth restriction. Growth should be compared to prior studies, if available.
- Fetal face — two eyes with lenses and upper lip
- Fetal stomach — presence, size, and situs
- Two kidneys — size, appearance, and location
- Urinary bladder — presence and size
- Umbilical cord insertion
- Fetal spine — cervical, thoracic, lumbar, and sacral
- Extremities — presence of two arms, legs, feet and long bones (digits not required but documented if possible)
- Maternal anatomy, noting pathology such as presence, size, and location of fibroids or ovarian cyst
Additionally, it’s crucial to note here that we should always be mindful in OB to incorporate the ALARA principle which is never more important than during our evaluation of a human fetus. Scanning at 100% power output is almost never needed, so practice prudently.
Finally, this report should comment on limitations for visualization as presented by fetal position and maternal body habitus. We then present to the reporting physician for interpretation.
We sonographers have an important task at hand…our physicians cannot manage what they don’t see. It is our ever-crucial responsibility to draw their attention to anything we suspect abnormal. Be familiar with pathology and differentials. Be prepared to discuss these with your reporting physician.
It is not our job to interpret findings, discuss those findings with our patients, or manage their care. But make no mistake…Yes! It is your job to diagnose. Your patients deserve a sonographer who is armed with the knowledge to detect subtle variations from the norm whenever possible. We all start somewhere. With a cautious mindset, we never stop learning.
My Final Impression
We aren’t expected to know it all. We are expected to find that which can be seen. The health and safety of expectant mom and baby-to-be…this has and always will remain the ultimate goal for ultrasound in obstetrics!