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What is prenatal screening and why should I get it?

Karyotype showing chromosomes of a normal male

Every pregnancy carries a small risk (far less than 1%) of having a baby with a significant birth defect. Some women, for example those older than 35 or those with a family history of a particular birth defect, have a higher risk.

While for most women the risk is small, knowing about birth defects during the pregnancy can be helpful. Some birth defects are severe and not compatible with life, some are associated with significant intellectual disability, and others may require surgery by a skilled pediatric surgeon very soon after birth to get the best outcome possible.

Prenatal screening is the use of blood tests from mom, sometimes combined with an ultrasound, to estimate the risk of some very significant birth defects. These tests do not tell you if you will have a baby with a birth defect, only if your risk is higher than expected based on your age. Prenatal screening will test risk for several birth defects, the most common being:

  • Down syndrome, a genetic abnormality that results from an extra chromosome 21, which means there is too much genetic material. This causes intellectual disability and several physical birth defects.
  • Trisomy 18, a genetic abnormality from an extra chromosome 18, also resulting in too much genetic material. Babies with trisomy 18 will have severe intellectual disabilities in addition to physical birth defects.
  • Neural tube defects, which are conditions that affect the developing brain and spinal cord (also called spina bifida). Some severe cases of neural tube defects are not compatible with life but other babies can live and even do very well if they have special neurosurgery very soon after they are born. In some cases surgery on the neural tube defect is even done during the pregnancy. Even with surgery, some children with neural tube defects may have problems moving their legs and working their bladder and bowel correctly.
  • Abdominal wall defects, which are conditions where the belly wall does not close probably as the baby is forming. Portions of the intestines (bowels) can protrude through the belly wall. Most babies with abdominal wall defects need to be delivered by c-section and also need surgery almost immediately after birth to place the intestines back in the belly. Some abdominal wall defects are associated with extra chromosome or other genetic conditions and this can affect the prognosis.
But why have one of these tests? Only if they will change what you will “do” during the pregnancy, and that means different things for different women.
Reasons to consider getting prenatal screening include the following:  
  • If knowing if your baby has a birth defect or special needs will help you prepare for his or her birth. For example, some babies with a birth defect may not be able to go to daycare and may need to be cared for at home. Some babies may need surgery in a larger, specialized medical center hundreds of miles from where you live. Finally, with some birth defects, knowing in advance helps your OB/GYN plan any special things that need to happen when you deliver (such as having a c-section or delivering in a hospital where the pediatric surgeon can assess your baby immediately after birth).
  • If you might consider a pregnancy termination if your baby has a significant birth defect.
Prenatal screening won’t tell you your baby has a birth defect, but tells your about your risk which will help you decide if you want more invasive tests or not (these tests carry a very small risk of miscarriage, so that is why everyone doesn’t just proceed right to invasive testing). It is important to understand that risk means different things to different women. For one woman a risk of 1/400 of having a baby with Down syndrome will seem very high and she may want further testing. However, I had a patient who felt a risk of one in two, meaning a 50% chance, of having a baby with trisomy 18 was low because there were still even odds that everything would be okay.
So a woman and her partner should consider prenatal screening if:
  • They want to know if their pregnancy carries a higher than expected risk of certain birth defects.
  • If they would like more information before having an invasive test that will tell then yes or no regarding these specific birth defects (some women go straight to more invasive testing based on risk related to age or family history and skip prenatal screening altogether).
  • If they understand that these tests are not perfect and can sometimes fail to identify a baby at risk.
Tomorrows post will cover the specific prenatal tests, when to have them, and what they mean. 



  1. Pingback: What are the different prenatal screening tests? « Dr. Jen Gunter - August 1, 2011

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