How accurate is prenatal testing?

This woman was told when she was pregnant that her baby’s MSAFP test (to check for spina bifida) came back abnormal; and an ultrasound showed that her baby “seemed to have a flat forehead” which could indicate anencephaly (no brain). She prepared herself for the birth of a baby who would be stillborn, die very quickly, or have life-long severe disabilities. They were wrong.

As an aside, I will say that spina bifida is not necessarily a severe disability. It usually is, but not always. I’ve known two people with spina bifida, and they were at the opposite ends of the spectrum, as far as the severity of the disease. One was a woman who lived in a nursing home all her life, her legs never grew beyond the size of a child’s, and she had to use a wheelchair to get around. The other was a childhood friend, and I didn’t know he had spina bifida until just a few years ago. He was completely normal, as far as I knew. When my mom said something to me about him having spina bifida, I was shocked; and she said that the only problem he had was that he couldn’t control his bowels — he had to wear diapers always “Just in case”; but he could control his bladder, and obviously could walk, run, and do whatever else kids do.


6 Responses

  1. You seem to be confused about the difference between screening tests and diagnostic tests. Screening tests identify people at risk for specific conditions. Screening tests, by definition and by design, will identify a much larger group of people than will actually have the condition at issue. Those people will then be tested with the diagnostic test which is more complicated, more inconvenient and more expensive than the screening test.

    So, for example, if we want to know which babies have Down’s syndrome, we could perform amniocentesis (the diagnostic test) on all 4 million pregnant women each year OR we can screen women with the AFP blood test (the screening test), identify the few tens of thousands that are at risk and offer those women amniocentesis. Which makes more sense: 4 million amnios or 4 million AFP blood tests followed by 40 thousand amnios? Obviously, performing the blood test and limiting the numbers of amniocentesis makes more sense from the point of view of safety, convenience and expense.

    The MSAFP test works exactly the way it was intended to work. It was never intended to make the diagnosis, so it makes no sense to criticize it because it does not accurately make the diagnosis.

  2. No, Dr. Amy, you are jumping to conclusions — I take it you did not read the woman’s full story. If you were to actually do that, you’d find that the ultrasound “confirmed” the MSAFP screen’s results, and the doctor said that the baby’s spine “seems to be open at the base.” The mother declined an amnio which would have given more accurate results, because she didn’t want to run the risk of a miscarriage, slight as it was. She felt that the only reason the doctor was pushing her to have it done would be to have an abortion if the amnio’s results came back with a diagnosis of spina bifida. Perhaps if her doctor had told her before she had the MSAFP that it was merely a screen and a large number of the “positive” results were false positives and further testing was required, she would have declined it altogether, sparing her the many months of grief and anguish of carrying a baby with presumed spina bifida. I rather suspect that there are many women who are being given such “screens”, assuming they are “tests” and are accurate, and their doctors don’t explain to them the nature of this in their long five-minute prenatal visit. This woman was one such; I know of another: a relative of a friend had experienced infertility, finally conceived with the help of some medicine or procedure, was hoping for a boy, found out her baby was a boy… and that it might have spina bifida. She had an abortion because the SCREEN said there was a chance of spina bifida. She assumed it was an accurate test, and that her baby definitely had it, and she had an abortion, killing her son that she had gone through who-knows-what to conceive, rather than raise an affected child. You don’t want me to criticize the tests and screens? Fine, I’ll criticize the doctors who don’t take the time to fully inform women about the limitations of these screens and tests. Don’t tell me that “it makes no sense to criticize it” when doctors don’t fully explain these tests and screens to women, and they make life-altering, and life-ending, decisions based on these screens and tests and ultrasounds, which all may ultimately be wrong, as was the case with this woman.

    I understand doctors have liabilities, and they try to reduce that as much as possible. I’d like to have some reforms of the laws that have allowed lawsuit abuses to take place — rendering million-dollar judgments against doctors who have done no wrong. I’ve heard of parents suing a doctor for “wrongful life” because *they* decided not to have prenatal testing done, and their baby had a birth defect of some sort, and they said that the doctor should have insisted that they do the testing; and if they had known beforehand that their baby had said birth defect, they would have had an abortion. That’s simply ridiculous! I’m suspecting that doctors are doing CYA in large part by offering, and even insisting and coercing women into getting these “elective” or “optional” tests and screens. That’s neither here nor there. If women get these tests and screens, they need to know beforehand their limitations. If it’s a screen, they need to hear that it’s designed to get everybody who may possibly be affected by it, which means that X% of “positives” will ultimately be negative. If it’s a test, they need to hear that it is more accurate, but there is a known false-positive or false-negative rate of X.

    This woman assumed the MSAFP was an accurate test; and the ultrasound seemed to confirm the spina bifida “diagnosis.” Shame on her doctor for not enlightening her — she had to find out long after her pregnancy was over, on her own. Had she been pro-choice, her perfectly normal baby might have been screened out of life.

  3. “If you were to actually do that, you’d find that the ultrasound “confirmed” the MSAFP screen’s results, and the doctor said that the baby’s spine “seems to be open at the base.”

    Evidently you have not heard of spina bifida occulta. You can read more about it at the Spina Bifida Association website (

    “By definition “Spina Bifida Occulta” means “hidden split spine.” The term is misleading because it is used to describe several conditions. The most frequently seen form is considered harmless and is simply a variant of normal vertebral (bone) anatomy. In this condition, parts of the bones of the spine called the spinous process and neural arch appear abnormal on a radiogram. Usually, the spinal cord, and spinal nerves are not involved. Isolated bony Spina Bifida Occulta (without an underlying spinal cord abnormality) does not lead to problems with the nervous system.”

  4. No, I’ve not heard of that. Thanks for the further information. Pity the doctor didn’t tell the woman that — it might have spared her months of grief and anxiety.

  5. He certainly could have told her that it was a possibility, but it is difficult to tell exactly what is going on before the baby is born.

  6. spina bifida occulta is a closed neural tube defect and the MSAFP test only screens for open neural tube defects so it’s not a consideration when looking at a MSAFP screen result. It may explain your male friends level of disability with a diagnosis of spina bifida though.

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