What's the best way to test for abnormal chromosomes during pregnancy?

Hi! I am 35 and nine weeks pregnant with my third child. My husband and I are weighing the merits of amniocentesis vs. chorionic villus sampling. We would like to know the relative rates of miscarriage of the two procedures. I have also heard that there is a possibility of limb malformation with CVS. Is this true? We also may skip both tests and rely on an AFP test and sonogram. Thanks for any info.

Deciding how to proceed can be difficult. I'm glad to hear that you're weighing your options. A woman's risk of delivering a baby with a significant chromosomal abnormality increases with age. At age 30 the risk is 0.26 percent, it increases to 0.56 percent at 35, and the risk is over 5 percent at age 45. Amniocentesis and chorionic villus sampling (CVS) are the two main ways to test for abnormal chromosomes during pregnancy. If a problem is found, parents can decide whether to terminate the pregnancy, or continue the pregnancy and be prepared for delivering a child with special needs.

Amniocentesis is is the gold standard and is the safer of the two procedures. Amniocentesis has an overall complication rate of 0.5 percent, including needle stick injuries to the fetus, infection, or leaking membranes. Nationwide, the risk of miscarriage from amniocentesis is only slightly higher than the risk of miscarriage in women who have not had an amniocentesis (3.5 percent vs 3.2 percent). In addition to checking the chromosomes, amniocentesis also checks the alpha-fetoprotein level which screens for major malformations such as neural tube defects.

The main drawback of amniocentesis is the timing of the test. This procedure is done in the second trimester at about 16 weeks and it takes a week to ten days to get the results. It can be hard waiting until 18 weeks to find out if all is well. Many centers now offer early amniocentesis which is performed closer to 12 weeks of pregnancy. The complication rate is a little higher than for traditional amniocentesis, but the results are available sooner.

The advantage of CVS is that it's performed earlier in pregnancy -- between ten and 12 weeks -- and the results are available sooner. When CVS was first introduced there were case reports of limb reduction malformations that may have been linked to very early procedures, around eight weeks. Nationwide, the risk of losing a baby after CVS is at least twice the risk of amniocentesis (range 1-4 percent). Additionally, about 10 percent of women have bleeding, spotting or cramping after CVS. Another drawback of CVS is that it does not test directly for AFP levels, so women who undergo CVS need to have a blood test for AFP in the second trimester.

While CVS has greater risks, taking those risks is appropriate for some women, especially those over 40 and also women who are very anxious. Since you have two healthy children and are only 35, amniocentesis may be a better choice. And if you prefer a risk-free approach, a detailed ultrasound and a triple test or AFP would be the next step. Only you and your husband know what's best for the two of you and for your family

Labels: Pregnancy health