In the 1980s, fetal ultrasound was a new technology. The key debate of the day was whether to screen all pregnant patients during the second trimester or only high-risk cases.
The question today is what not to miss on ultrasound, especially during the first trimester.
“An ultrasound scan in the first trimester can identify major abnormalities early in pregnancy,” said Stephen T. Chasen, MD, FACOG, professor of OB/GYN at Weill Cornell Medical College. “Many structures can be diagnosed below 14 weeks. If you can identify a condition early, the patient can pursue appropriate testing and follow-up scanning to make more informed decisions regarding the pregnancy. Earlier diagnosis can lead to earlier abortion, which is safer, more private and more available.”
Dr. Chasen reviewed first- and second-trimester ultrasound findings during a Monday morning clinical seminar. The issue is not whether to perform ultrasound, he said, but knowing what to look for during a scan.
The temptation is to simply follow guidelines from professional associations, institutions and payers. But guidelines are nothing more than recommendations. And recommendations can trail behind good clinical practice.
“Ultrasound exams should not be limited to a checklist from the insurer,” he said. “We are spending real time with the patient and should be doing more than checking off the boxes. If you have the patient on the table, you don’t want to miss abnormalities in the face, the heart, the extremities, the genitalia. We always go beyond the basic list.”
Take the typical second-trimester recommendation for chest and heart examination. The basic exam recommends a four-chamber view of the fetal heart. A four-chamber view is a good start, Dr. Chasen said, but it is only the beginning.
“If you don’t examine the outflow structures, the ascending aorta, pulmonary trunk and bifurcation/ductus arteriosus, you will miss important anomalies,” he said. “A four-chamber view won’t show things like the tetralogy of Fallot, truncus arteriosus or a transposition of the great vessels. You’re not going to find what you are not looking for.”
Major anatomic anomalies and anatomic clues can be readily visualized during the first trimester. Nuchal edema is a marker for other structural anomalies and is more likely to be identified during the first trimester than later in pregnancy.
A simple transverse view of the first trimester fetal brain can identify the lateral ventricles and the midline. The shape and integrity of the skull can be evaluated. So can the presence of anencephaly or acrania, holoprosencephaly and large cephalocele, structural defects associated with extremely poor neurologic outcomes.
Major facial structures can be evaluated during the first trimester, including the profile, orbits and possibly the lips.
“Take a good look at the orbits,” Dr. Chasen said. “You don’t want to miss severe hypotelorism or cyclopsism. There should be two orbits, not one.”
Chest and abdomen exams can be equally telling. Most of the major organs are developing during the first trimester, making it possible to evaluate placement of the heart and lungs as well as any mediastinal shifts. It is also possible to evaluate the position and size of the stomach bubble, umbilical cord insertion, ventral wall and urinary bladder.
Most spinal abnormalities are not visible during the first trimester, but the limbs are clearly visible.
“You want to make sure there are two hands and two feet,” Dr. Chasen said. “And you don’t want to miss any major limb reduction defects or severe skeletal dysplasia. You can suspect polydactyly, but you will miss most of it until the second-trimester scan.”