Like much of medicine, ultrasound diagnosis of fetal anomalies is both a science and an art. Part 1 of this article will detail, within the text and with images, the anomalies that should not be missed when performing ultrasound during the first and second trimesters of pregnancy. Part 2, in a future issue, will deal with multifetal gestations. This series will discuss and show the appearance of anomalies in the first and second trimesters, and in multifetal gestations (anomalies unique to multiples) to enhance the ob/gyn’s knowledge of pitfalls that can lead to errors in diagnosis.
Ultrasound and prenatal diagnosis of structural fetal anomalies
Ultrasound can identify the majority of major structural fetal abnormalities. Prenatal diagnosis can lead to improved outcomes by ensuring that delivery occurs in a hospital with the necessary personnel to manage newborns who may require surgery or other specialized care. In rare cases, prenatal diagnosis can lead to fetal intervention, although most anomalies do not require any treatment in utero. Some structural anomalies are associated with genetic conditions, and recognition can lead to prenatal genetic diagnosis. In cases in which prenatal diagnosis reveals a major structural abnormality, some patients may choose to terminate the pregnancy.
Patients who present early for pre- natal care typically undergo 2 ultrasound fetal assessments. At 11 to 14 weeks, when nuchal translucency (NT) is measured as a component of Down syndrome screening, a brief fetal anatomic scan is performed. A first-trimester ultrasound is a valuable tool to confirm viability, rule out multiple pregnancy, and evaluate anatomy, even if genetic screening is not desired. The main ultrasound used to screen for structural anomalies is in the second trimester, generally at 18 to 20 weeks
In the first trimester, some major anomalies can be diagnosed or excluded. In other cases, there may be findings that are not diagnostic but that may suggest a structural anomaly. In these patients, second-trimester ultrasound before 18 weeks can lead to earlier prenatal diagnosis. Abnormal NT in a fetus with a normal karyotype is associated with a higher rate of structural abnormalities. When NT is abnormal, it is reasonable to assess fetal anatomy early in the second trimester, as some structural anomalies can be identified prior to 18 weeks’ gestation.
Late in the first trimester, the brain can be imaged in the transverse plane, identifying both hemispheres and midline structures. Major anomalies that can be identified include anencephaly (absent skull and brain; Figure 1), acrania (absent skull), and holoprosencephaly (no division into separate hemispheres, with absence of midline structures; Figure 2). If a transverse view through the fetal brain identifies a normal midline, representing the falx cerebri, and lateral ventricles, these 3 conditions can be excluded. A large skull defect (cephalocele; Figure 3) can be identified as well.
In the same image used to assess NT, the profile can be evaluated. A profile view can identify a small mandible, or micrognathia (Figure 4). Large median or bilateral cleft lip can also be suspected based on profile views. Coronal imaging can identify the orbits, and large clefts may be visible. Rarely, facial masses representing teratomas or lymphangiomas may be visible.
NT measurements require proper fetal position, image magnification, and caliper placement. All individuals measuring NT as a component of screening for genetic abnormalities must undergo the requisite training, credentialing and quality review.1 An extreme variant of abnormal NT is the cystic hygroma, characterized by midline septations and edema extending to the fetal thorax. While studies looking at outcomes of this condition have identified high rates of genetic and structural abnormalities, it is not clear that the prognosis is poorer compared to fetuses with comparable NT measurements not categorized as cystic hygroma.2
Evaluation of the chest is very limited in the first trimester. The heart should be visible in the midline, and lung tissue should be present on both sides. Mediastinal shift can represent evidence of a chest mass or diaphragmatic hernia, although those are uncommon diagnoses early in pregnancy. Late in the first trimester, it should be possible to distinguish the left from the right side of the heart, and normal situs can be verified. While structural cardiac anomalies rarely can be suspected, diagnosis before the second trimester is uncommon.
An abnormal-appearing cardiac axis may reflect underlying structural abnormalities.3
In the first trimester, visible structures include the ventral wall, umbilical cord insertion, stomach bubble, and urinary bladder. The most common abnormalities identified are the ventral wall defects omphalocele (Figure 5), gastroschisis (Figure 6 and discussed in detail in our February 2017 issue), and more extensive defects including body-stalk anomaly and limb-body-wall complex. Physiologic midgut herniation is common prior to 12 weeks’ gestation and should not be confused with an omphalocele. An enlarged urinary bladder (megacystis; Figure 7) can represent early evidence of bladder outlet obstruction. Kidneys are generally not imaged at < 14 weeks, and because amniotic fluid does not consist primarily of fetal urine until the second trimester, absent or dysfunctional kidneys will not result in oligohydramnios earlier in pregnancy.
The spine is not easy to evaluate in the first trimester, and most abnormalities will go undetected. Spina bifida involving multiple levels can be suspected in rare cases, as can large masses such as sacrococcygeal teratoma.