Risks of Birth Defects

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Birth defects are an infrequent but major source of childhood illness. Although many families worry that their infant will have a birth defect, for most the risk is low. Only about 2 percent of babies have a recognizable birth defect; about half of these are caused by known genetic problems. Fortunately, many of these children also have satisfactory outcomes; however, some require special care before, during, and after delivery.

Risks

The risk of an infant having a birth defect varies according to the mother's age and the medical history of both parents. Women less than 35 years of age at delivery with no personal or family history of birth defects are at low risk. At your initial prenatal visit, your doctor will evaluate your risk of having a baby with a birth defect. You should also receive information about the available tests for further evaluating your risk. Despite the absence of risk factors, some low-risk women do have babies with birth defects. Prenatal screening tests are used to assess the risk of having an infant with a birth defect in women with low risk. Diagnostic tests help confirm or exclude the presence of birth defects before delivery in women with increased risk.

If a genetic or other problem is present, you and your medical caregivers should discuss your options. These problems can influence how you deliver your baby (C-section), where you deliver (in a hospital with a newborn intensive care unit), and who will be present at delivery (pediatric specialists).

Factors Increasing Your Risk

Your doctor is in the best position to evaluate your risk of having an infant with a birth defect; however, you can estimate your own risk by reviewing the following risk factors:

• Maternal age greater than 35 years at delivery increases the risk of Down syndrome (extra chromosome number 21) and other chromosomal problems in the baby. There is a gradual increase in chromosome problems with increasing age. This increase accelerates after age 35. The age of 35 is used as a cutoff value for more detailed testing. Women over 35 at delivery are usually offered amniocentesis to check for a chromosome problem in the fetus. For example, the risks of having an infant with Down syndrome or other similar chromosomal problem (per 1,000 deliveries) are as shown below.

  Maternal Age at Delivery and Risk of Down Syndrome or Similar Chromosomal Birth Defect
  Maternal Age (Years)	Chromosomal Birth Defect *
  25-29	2
  30-34	3
  35	5
  36	6
  37	8
  38	10
  40	16
  41	20
  45	50

  * Data from combined sources.

• Having had a previous fetus with genetic problems or birth defects may increase your risk of having a subsequent infant with a problem. The magnitude of the risk depends on the nature of the previous problem. If a previous fetus had a known genetic disease such as cystic fibrosis, subsequent children have a 25 percent risk (1 out of 4) of also having cystic fibrosis. For other problems, the risk of having a child with a similar problem could be as low as 3 out of 100.

• A family history of genetic problems in you, your spouse, or anyone in your extended family may increase your risk of having an infant with genetic problems.

• Abnormal screening test results suggest that your baby may face a higher risk than expected by age or history. Screening blood tests or ultrasound can identify some of the low-risk women whose fetuses have genetic or other problems. A problem with these tests is they sometimes give falsely positive results; they indicate a high risk for genetic problems even when those problems are not there. This can cause considerable anxiety for expectant parents. Remember that these are risk-assessment tools, not diagnostic tests. Likewise they may not indicate a problem when one does exist. If there is an abnormal screening test, additional testing and counseling are offered to address your concerns.