Question: "I am 38 years old and pregnant with my second child. I have heard there are extra screening tests for me such as amniocentesis. But I've also heard that it can be risky. Do I need to have one?"
Whether or not to have prenatal diagnostic testing is a personal decision that all pregnant women, regardless of their age, need to make for themselves.
The older our ovaries get, the greater the likelihood of a chromosomal abnormality. (The egg and sperm each carry 23 chromosomes, giving each cell in the fetus 46 chromosomes altogether.) The majority of pregnancies with an atypical number of chromosomes will end in miscarriage. However a few, most commonly Trisomy 21 (otherwise known as Down Syndrome in which there is an extra chromosome number 21), will continue to term. Women age 35 and older are considered “advanced maternal age”. 35 was chosen as the "threshold" age because the risk of having a baby with Trisomy 21 at age 35 is equal to the risk of miscarriage from an amniocentesis - approximately one in 250. But otherwise, there is nothing magical about age 35. Our risk of having a chromosomally abnormal fetus increases as we age but fortunately there are screening and diagnostic tests we can use to find out if the baby is healthy.
Each state has its own prenatal diagnostic program to screen for birth defects. For instance, in California you can elect to have a blood test done between 10 and 14 weeks of the pregnancy. The results of this blood test can be combined with a measurement of the fetus’ neck thickness (nuchal translucency) which gives a screening risk for both Trisomy 21 and Trisomy 18. Between fifteen and twenty weeks, another blood test is performed which is combined with the other two measurements to give a final screening number (Sequential Integrated Screening). This test will pick up pregnancies affected by one of these age related chromosomal abnormalities in about 90 percent of cases. One can also choose to have only the first and second trimester blood tests (Serum Integrated Screening) or only the second trimester blood test (Quad Marker Screening) though the test will be less sensitive. Screening tests will let you know if there is an increased or decreased risk. They will not give you a final answer but they also do not have complications associated with diagnostic tests.
A new type of screening called "Noninvasive Prenatal Testing" or NIPT was introduced recently and is offered by several commercial companies. Essentially a sample of the mother's blood is taken after the 10th week of pregnancy. Chromosomes reside in the cell nucleus but when cells break apart, fragments of the DNA can be found in the bloodstream. With NIPT, fragments of the fetus' DNA are separated out from fragments of the mother's DNA and analyzed. The test looks for abnormal amounts of fetal chromosome 21 as well as 18, 13, and gender chromosomes. (In other words, you can find out the sex of the baby along with the results if you wish.) It is extremely sensitive in picking up these chromosomal abnormalities. At this point, this screening option is only recommended for women in a high risk group -- age over 35, state screening or nuchal translucency results in the high risk category, or another factor such as a previously affected pregnancy.
If any of these screening tests return with an increased risk or if you are in a high risk category (over 35, for example) and would feel more comfortable with definitive results, you can elect to have a diagnostic test which will give you the actual chromosomes of the baby. Chorionic villus sampling (CVS) is performed between nine and fourteen weeks and involves taking a tiny sample of cells from between the baby and the placenta. An amniocentesis is another diagnostic test and is done between sixteen and twenty weeks . A needle is carefully placed into the uterus under ultrasound guidance and amniotic fluid with cells the baby has shed is withdrawn. In both of these procedures, the cells are then grown and the chromosomes analyzed with results available in two weeks. Neither of them hurt - they feel like a pinch or a shot. While they offer more conclusive results than the screening tests, they also incur a small amount of risk such as bleeding, infection, or miscarriage of the pregnancy. It is also routine to have an ultrasound to check the baby’s anatomy sometime between eighteen and twenty weeks.
Ultimately, you and your partner should discuss your feelings about screening and diagnostic tests to check for birth defects. Some couples will choose not to have any screening done while others will strongly desire early tests. Even if you do not feel that knowing of a chromosomal irregularity will change how you proceed with your pregnancy, the information may help you prepare for your baby. All of these options should be discussed with your obstetrician. The decision of how to proceed is yours.