Spina bifida is typically diagnosed before birth through a combination of blood tests and imaging, most often during the second trimester. In some milder cases, it may not be detected until after delivery or even later in childhood. The diagnostic path depends on the type of spina bifida involved: open forms, where the spinal canal is exposed, are nearly always caught on prenatal ultrasound, while closed or skin-covered forms can be much harder to spot.
Blood Screening in the Second Trimester
The first hint of spina bifida often comes from a routine blood test offered between weeks 15 and 20 of pregnancy. This test measures a protein called alpha-fetoprotein (AFP), which is produced by the developing baby and passes into the mother’s bloodstream. When a baby has an open neural tube defect like spina bifida, AFP leaks through the gap in the spine into the amniotic fluid and then into the mother’s blood, raising levels above the normal range.
A high AFP result does not confirm spina bifida on its own. It simply flags an increased risk and triggers further testing, usually a detailed ultrasound. AFP can also be elevated for other reasons, including carrying twins or an inaccurate estimate of gestational age, so most people with a high reading turn out to have perfectly healthy pregnancies.
Prenatal Ultrasound: The Primary Diagnostic Tool
A detailed ultrasound during the second trimester is the most reliable way to diagnose spina bifida before birth. For open forms of the condition, studies have shown that sensitivity and specificity are close to 100%, meaning nearly all cases are detected. The few that slip through screening tend to be skin-covered types like small meningoceles or fatty tissue malformations, which don’t produce the same visible markers.
Sonographers look for a set of well-established signs. On a cross-sectional view of the spine, normal vertebrae form a closed circle, while in spina bifida the arch appears U-shaped with a bulging sac visible behind it. But some of the most telling clues come from the baby’s skull and brain rather than the spine itself. A characteristic scalloping of the frontal bones, known as the “lemon sign,” appears in virtually all open cases during the second trimester. The cerebellum, a structure at the base of the brain, may curve forward into a crescent shape called the “banana sign,” or it may not be visible at all. Enlargement of the fluid-filled spaces in the brain (ventriculomegaly) is also common, appearing in roughly half of affected fetuses.
These cranial markers are important because they can alert a sonographer to spina bifida even when the spine itself is difficult to image clearly due to fetal position.
Amniocentesis for Confirmation
When blood tests or ultrasound findings are uncertain, amniocentesis can help clarify the picture. A small sample of amniotic fluid is drawn and tested for two substances. The first is AFP, which is more concentrated in the fluid surrounding the baby than in the mother’s blood and therefore easier to measure accurately. The second is an enzyme normally found in spinal fluid. When this enzyme appears at elevated levels in the amniotic fluid, it strongly suggests an open neural tube defect.
In early studies, testing amniotic fluid for this enzyme correctly classified nearly all pregnancies, including several that the AFP test alone had gotten wrong. A specialized follow-up test on the same fluid sample was able to correctly resolve every remaining ambiguous case. Today, amniocentesis is less commonly needed because high-resolution ultrasound catches most cases, but it remains a valuable backup when imaging is inconclusive.
Fetal MRI for Surgical Planning
Once spina bifida is confirmed on ultrasound, some families are offered a fetal MRI. This scan provides detailed images of the brain and spinal cord in soft tissue, which can help surgeons plan for prenatal or postnatal repair. However, research comparing the two imaging methods found they are equally accurate at identifying the level of the spinal defect, with both agreeing with postnatal findings about 80% of the time.
Both techniques also share a limitation: in roughly one out of five cases, they misjudge the exact spinal level by two or more segments. This matters because the location of the defect along the spine is the strongest predictor of which nerves will be affected and what kind of mobility or bladder function the child can expect. For this reason, doctors are careful not to make firm predictions about neurological outcomes based on prenatal imaging alone.
How Milder Forms Are Found After Birth
The mildest form, spina bifida occulta, involves a small gap in one or more vertebrae without any exposed spinal tissue. It produces no visible sac on the baby’s back and doesn’t leak AFP into the amniotic fluid, so it’s invisible to standard prenatal screening. Many people live their entire lives without knowing they have it.
In newborns, certain skin markings over the lower spine can signal an underlying problem. These include a tuft of hair, a small dimple, or a birthmark. Not every dimple or birthmark is meaningful, but when a doctor notices one in a suspicious location, they’ll order imaging to check the spinal cord beneath it.
For infants, spinal ultrasound is an effective and noninvasive first step. It works best between about 4 and 6 months of age, when the bony structures of the spine are still soft enough for sound waves to pass through but developed enough to give clear measurements. Before 3 months, the anatomy can be too small to assess reliably, with one study finding that key structures couldn’t be measured in over 80% of newborns under one month old. After about 6 months, bone growth begins to block the ultrasound beam, and MRI becomes the preferred option for a definitive look at the spinal cord.
Distinguishing Types on Imaging
Not all spina bifida looks the same, and the distinction between types matters for treatment planning. On ultrasound, a meningocele appears as a thin-walled, fluid-filled sac protruding from the spine. It contains only the protective membranes surrounding the spinal cord, not nerve tissue itself, so outcomes tend to be better. A myelomeningocele, the more serious form, involves spinal cord tissue or nerves extending into the sac. This is the type most commonly associated with leg weakness, bladder problems, and the brain changes that produce the lemon and banana signs.
In closed or skin-covered defects, the overlying skin is intact, which is why they don’t trigger elevated AFP levels and are harder to catch prenatally. The Danish national screening data illustrate this gap clearly: among cases missed by routine ultrasound, nearly 80% turned out to be skin-covered forms rather than open defects.