There are many options for screening for Trisomy 21 (Down's syndrome), Trisomy 18 or Trisomy 13 during pregnancy. These chromosomal abnormalities are the most common genetic causes of mental retardation. Infants born with Trisomy 18 or 13 rarely survive past one year of age, children born with Trisomy 21 will have developmental problems but can survive well into adulthood. A pregnant woman's chance of having a fetus with any of these conditions increases as a woman ages due to the aging of the oocytes (or eggs).
The genetic screening options currently available include integrated screening or sequential screening. Both involve a blood test and ultrasound in the first trimester with another blood test in the second trimester. The overall detection rate is approximately 90% for both options. The difference between the two options is that a patient is sequentially notified of results with the sequential screen (i.e.-a risk is calculated after the first trimester ultrasound and blood test and the patient is notified, then the risk is recalculated after the second trimester test and the patient is notified.) The integrated screening combines both results and calculates one risk which the patient is notified of during the second trimester. The advantage of the sequential screen is being informed in the first trimester if the woman has an elevated risk. A woman may then choose to proceed to invasive testing(more details on invasive testing later) for confirmation of a fetus with a chromosomal abnormality. The advantage of the integrated screening is that it has the highest detection rate and lowest false positive rate but a woman would not be informed until the second trimester.
Invasive testing for chromosomal abnormalities include chorionic villus sampling and amniocentesis. The tests obtain fetal cells, grow the cells in a culture, and then test for the chromosomes directly. The accuracy is 99%. Chorionic villus sampling is done in the first trimester. It involves using a needle to obtain a portion of the placenta. The needle can go either through the cervix or the abdomen. The pregnancy loss rate associated with chorionic villus sampling is approximately 1 in 100 procedures. The advanatage of the chorionic villus sampling is that it allows a women to know whether her fetus has a chromosomal abnormality within the first trimester.
Amniocentesis is done during the second trimester and involves a needle into the woman's abdomen and obtaining amniotic fluid. The fetal cells are then grown in culture and chromosomal results are obtained. The pregnancy loss rate for amniocentesis is 1 in 300-400 procedures. The advanatage of the amniocentesis is the lower pregnancy loss rate but with the disadvantage of not having results until the second trimester.
I recommend for all my patients to consider genetic screening. I think it is helpful to know if a woman has a higher risk of having a child with a chromosomal abnormality because I think it is useful to be prepared with that information before the fetus is born. These tests can also indicate if there is possibly something wrong with the placenta even if the chromosomes are normal which would require closer monitoring during pregnancy. Most insurance plans cover testing as a basic obstetrical practice.
Here are some additional links regarding genetic screening:
http://www.uptodate.com/contents/patient-information-should-i-have-a-screening-test-for-down-syndrome-during-pregnancy?source=see_link
http://www.uptodate.com/contents/patient-information-chorionic-villus-sampling?source=see_link
http://www.uptodate.com/contents/patient-information-amniocentesis?source=search_result&search=amniocentesis&selectedTitle=2%7E150
http://www.acog.org/publications/faq/faq165.cfm
