How to Calculate the Risk of Aneuploidy

The risk of aneuploidy increases with maternal age.

Women older than 35 have an elevated risk of delivering a child afflicted with aneuploidy, the condition of having more than or fewer than the normal number of chromosomes. Down syndrome, Turner's syndrome and Klinefelter syndrome are three of the more common forms of aneuploidy. Typically caused when something goes awry during cell division, aneuploidy is the primary known cause of fetal miscarriage. One in 160 live births produces an aneuploid infant, but with proper and early prenatal screening, the obstetrician can estimate a woman's approximate risk.

Instructions

1. 

   When cells don't have the correct number of chromosomes, aneuploidy is the result.

   Visit your obstetrician as early in the pregnancy as possible. Explain your concern to her, particularly if you're 35 or older. Ask her to order screening tests for aneuploidy; genetic testing must be performed at specific times and intervals. According to the Reproductive Genetics Health Institute, the risk of aneuploidy is one in 385 for a thirty-year-old woman, one in 63 for a thirty-five-year-old woman and one in 19 for a forty-five-year-old woman.

2. 

   Ultrasound can help diagnose Down syndrome.

   Request a blood test that includes PAPP-A and iITA; decreased levels are common in Down syndrome. Ask for an ultrasound for nuchal translucency (NT); this measures the fluid under the fetal neck skin, an indicator of aneuploidy if elevated. (Have the ultrasound at 8 to15 weeks, and again at 28 to 32 weeks.)

3. Have a chorionic villus sampling (CVS) test performed at 10 to12 weeks. A CVS test collects cells from the placenta using a catheter inserted into the cervix, or using a needle through the abdomen. CVS results can take up to two weeks.

4. 

   Keep your prenatal doctor visits.

   Have an amniocentesis performed, especially if you're older than 35, in the first trimester or the second trimester, or both, according to your obstetrician's assessment and advice. The test involves passing a thin needle through your abdomen, into the uterus. About an ounce of amniotic fluid will be aspirated for genetic testing.

5. 

   Blood tests are not painful.

   Have a "triple test" at 15 to18 weeks. This trio of blood tests measures alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin. A concurrent ultrasound to determine fetal age, combined with the triple test, gives your obstetrician a more sensitive range of data with which to calculate the risk of aneuploidy.

6. 

   Share your thoughts and fears with your doctor.

   Discuss all test results with your obstetrician, and ask for clarification of anything you don't completely understand. Your obstetrician will review your medical history and test results, and from them calculate your risk, if any, of aneuploidy. Ask for recommendations for counselors or support groups, if needed.

Tips & Warnings

• Educate yourself about genetic testing and the possible results. Knowing what testing involves, and what the results mean, will reduce anxiety and fear.

• The need, timing and frequency of medical tests are ultimately the decision of the patient and her obstetrician. These are merely guidelines and not intended to replace medical advice from a licensed physician.

• Even with sophisticated testing methods, inherent false positives and false negatives can and do occur. Some procedures, such as amniocentesis and chorionic villa sampling, may carry a slight risk of inducing miscarriage.

• Approximate, but not exact, risk can be calculated. A woman's age, weight and ethnicity, and whether she smokes or is insulin-dependant, introduce variables that affect results.

• Be prepared to face ethical and moral dilemmas with positive aneuploidy results. Some parents don't want testing and are prepared to accept any contingency; others may decide to terminate the pregnancy.

References

• AnEUploidy.eu: Aneuploidy at a Glance
• Colorado State University: Aneuploidy and Deletions
• Reproductive Genetics Institute: What is Aneuploidy?
• American Family Physician: Down Syndrome: Prenatal Risk Assessment and Diagnosis
• Wiley Online Library: Calculating Patient-speci?c Risk for Down Syndrome: Does Software Matter?
• American Pregnancy Association: Chorionic Villus Sampling

Resources

• National Library of Medicine: Genetics Home Reference
• NEJM: Prenatal Screening for Aneuploidy
• HiRiskOB.com: Glossary of Tests
• Yudkowsky: An Intuitive Explanation of Bayes' Theorem