Prenatal ultrasound uses high-frequency sound waves to create images of a developing fetus, making it an invaluable screening tool for physical development. The Level 1 and Level 2 anatomy scans are designed to identify large-scale or gross structural anomalies, such as major heart defects, missing limbs, or severe neural tube defects. Because this technology relies on imaging physical structures, it is inherently limited when a condition does not result in a visible change to the organ’s size, shape, or density. Consequently, any condition that operates at a functional, cellular, or molecular level, without altering the anatomy, will not be detectable by ultrasound.
Defects Based on Cellular Function
Ultrasound technology is unable to detect birth defects that are primarily biochemical or metabolic, as these conditions do not typically manifest as physical changes in organ structure during the prenatal period. These disorders stem from faulty enzymes or cellular processing errors that impair the body’s ability to convert food into energy. For instance, inborn errors of metabolism, such as Phenylketonuria (PKU), involve a deficiency in the enzyme phenylalanine hydroxylase needed to break down phenylalanine. This failure occurs at a microscopic level, leaving the baby’s physical structure appearing completely normal on an ultrasound screen.
Another example is congenital hypothyroidism, where the thyroid gland either fails to develop properly or cannot produce sufficient thyroid hormone. This hormonal deficiency is a functional problem that does not cause a visible alteration to the fetal thyroid gland’s shape or size during gestation. While severe metabolic disorders can sometimes lead to secondary, non-specific findings like intrauterine growth restriction, the primary enzyme defect itself remains invisible to imaging technology.
Conditions Affecting Nervous System Development
Many complex conditions affecting the nervous system are missed because the brain’s gross structure appears correctly formed on imaging, despite compromised function. Conditions like mild cerebral palsy, certain developmental delays, and intellectual disabilities often involve subtle abnormalities in neuronal connections or cellular migration, not changes in overall brain size or shape. Ultrasound can effectively detect severe structural defects like anencephaly or spina bifida, but it cannot resolve the microscopic wiring issues that lead to functional impairment.
Sensory conditions such as deafness or blindness are also frequently undetectable unless they are linked to a severe structural anomaly of the ear or eye. Congenital deafness, for example, is often caused by defects in the tiny hair cells of the inner ear or the auditory nerve pathways, which are far too small for ultrasound resolution to capture. Similarly, blindness not caused by a visibly malformed eyeball, but rather by issues with the optic nerve or visual processing centers, will not be apparent on a prenatal scan.
Structural Anomalies That Are Minor or Late-Developing
Even when a defect is structural, limitations in ultrasound resolution, timing, and image quality mean that many anomalies can still be missed. Very small structural defects, such as a minor ventricular septal defect (VSD)—a small hole between the heart’s lower chambers—may be too minute to be reliably resolved. A VSD only a few millimeters in diameter can easily fall below the threshold of detection, particularly in a moving fetus. The accuracy of a scan is also highly dependent on the fetus’s position and the amount of maternal tissue the sound waves must penetrate.
Other structural issues may simply not develop until late in the third trimester or even after birth, making them invisible during the standard mid-pregnancy anatomy scan. Subtle skeletal anomalies or issues with the urinary tract, such as mild ureter widening, may not fully manifest or become large enough to be seen until later in gestation. If the defect is not present or is too subtle at the time of the scan, advanced equipment cannot identify it.
The Role of Other Prenatal Screening Methods
The limitations of ultrasound in detecting non-structural and microscopic conditions highlight the necessity of other prenatal screening methods. Non-Invasive Prenatal Testing (NIPT) is a blood test performed on the mother that screens for common chromosomal abnormalities like Down syndrome (Trisomy 21) by analyzing cell-free fetal DNA. This test provides genetic risk information that ultrasound cannot offer, as most chromosomal conditions do not have specific, visible structural markers.
For definitive diagnoses of genetic or metabolic disorders, diagnostic procedures like Chorionic Villus Sampling (CVS) or amniocentesis are employed. CVS is performed earlier in pregnancy, and amniocentesis is performed later; both involve obtaining tissue or fluid samples that contain fetal cells. These samples are analyzed in a laboratory to confirm a chromosomal abnormality or a specific genetic mutation that causes a metabolic defect. These non-imaging tests directly assess the fetus’s genetic and biochemical makeup, providing the functional information that ultrasound technology is unable to capture.