All pregnant women have a risk of birth defects, including Down syndrome and other chromosome abnormalities. Most women don’t know what their risk is, however, and may be surprised to learn what it is. The overall risk for all maternal ages is approximately 1 in 500 pregnancies at the time of a 2nd trimester ultrasound, based on the maternal age distribution. This risk has increased from 1 in 700 two decades ago, reflecting the trend toward delayed child birth.. Of course, the individual risk varies with the individual patient’s age. The risk also decreases with gestational age because there is a certain ‘attrition rate’ due to the increased risk of spontaneous intrauterine death among fetuses with major chromosome abnormalities.
The table below summarizes the risk of fetal Down syndrome based on both maternal age and gestational age. The risk is reported as 1 in __. For example, a 30 year old woman has an approximate risk of 1:703 at 16 weeks.
Gestational Age (Weeks)
Age 10 12 14 16 20 40
20 983 10681140 12001295 1527
25 870 946 100910621147 1352
30 576 626 668 703 759 895
31 500 543 580 610 658 776
32 424 461 492 518 559 659
33 352 383 409 430 464 547
34 287 312 333 350 378 446
35 229 249 266 280 302 356
36 180 296 209 220 238 280
37 140 152 163 171 185 218
38 108 117 125 131 142 167
39 82 89 95 100 108 128
40 62 68 72 76 82 97
41 47 51 54 57 62 73
42 35 38 41 43 46 55
43 26 29 30 32 35 41
44 20 21 23 24 26 30
45 15 16 17 16 19 23
BIRTH DEFECTS
ALL pregnancies are at risk for birth defects, regardless of the patient’s age, health status, previous history, family history, or life style. Birth defects are not uncommon, affecting at least 2% of pregnancies. Fortunately, most birth defects are minor while major birth defects are much less common. However, a number of serious birth defects occur in the range of 1 in 1000 pregnancies. Screening tests are available that can detect the vast majority of these serious birth defects. For parents of affected infants, a number of parent support groups are available.
DIAGNOSTIC & SCREENING TESTS
Prenatal testing is available for many important birth defects. These tests may be considered either “diagnostic” or “screening” tests. Diagnostic tests can tell whether a fetus has a particular condition with certainty but may provide limited information about other types of birth defects. Also, some diagnostic tests such as amniocentesis carry a small risk of complications that could result in fetal death. Screening tests are not as accurate as diagnostic tests but do help to identify patients at risk who might benefit from a diagnostic test. Screening tests will result in both “false positive” results and some missed cases. A primary advantage of screening tests is they are not invasive and so do not carry a risk to the fetus. When optimally performed and interpreted by experts, ultrasound can both “screen” for a variety of major defects and can also make a specific diagnosis. For example, a quality ultrasound with fetal anatomic survey should be able to identify virtually all cases of spina bifida and neural tube defects, whereas AFP testing would be considered a screening test. For other types of defects – particularly Down syndrome – screening tests such as ultrasound may not be diagnostic themselves but can help identify those patients at highest risk who might consider genetic amniocentesis.
The following is a partial list of the types of conditions in which screening and diagnostic tests are available.
SCREENING TESTS
DIAGNOSTIC TESTS
DOWN SYNDROME TESTING
Maternal Age
Combined Screen for fetal aneuploidy (11-14 weeks, ideally 11-12 weeks)
Biochemical Testing First or Second Trimesters (11-14,15-20 weeks
Integrated Screen (First trimester screen + 2nd trimester screen with one result)
Sequential or Step wise screen (First trimester screen + 2nd trimester screens with two results).
Second trimester genetic sonogram (15-20 weeks)
Genetic amniocentesis (14-20 weeks)
Chorionic villus sampling (CVS) (10-13 weeks)
SPINA BIFIDA & NEURAL TUBE DEFECTS
AFP screening (80-90% detection rate).
Level 1 office ultrasound (in reality, virtually all spinal defects should be detected or suspected on any fetal screening survey)
Targeted or level 2 ultrasound
Amniocentesis with AFP and acetylcholinesterase testing (nearly diagnostic)
CLEFT LIP / PALATE
A carefully performed level 1 office ultrasound can detect or suspect some cases of cleft lip/ palate
Detailed ultrasound with fetal survey may be diagnostic and a screening test.
OTHER STRUCTURAL BIRTH DEFECTS
Level 1 office ultrasound can detect or suspect many major structural birth defects
Detailed ultrasound with fetal survey can detect the majority of important birth defects and in many cases is also diagnostic.
TAY SACHS DISEASE
Blood tests can determine whether the parents carry the genetic mutation responsible for Tay-Sachs Disease. A Tay-Sachs carrier has one normal gene for hex A and one Tay-Sachs gene. The carrier does not have the illness and leads a normal, healthy and full life. However, when two carriers become parents, there is a one-in-four chance that any child they have will inherit a Tay-Sachs gene from each parent and have the disease. In that situation, diagnostic tests may be performed on the fetus.
Detailed ultrasound with fetal survey can detect the majority of important birth defects and in many cases is also diagnostic.
OTHER JEWISH RELATED CONDITIONS
Blood tests can screen for other conditions that are more common among Jewish patients as part of a “Jewish” panel
Amniocentesis or CVS may be diagnostic for some of these conditions
SICKLE CELL DISEASE
Blood tests on the parents will identify people who have either sickle cell trait or a form of the disease, as well as a number of other less common inherited hemoglobin abnormalities. If both parents have the sickle cell trait, the fetus will have sickle cell disease in 1/4 of cases and this can be tested for as a diagnostic test.
If both parents are carriers of Sickle cell, the fetus will have Sickle cell disease in 1/4 of cases. This can be tested for prenatally to determine for certain whether the fetus is affected.
THALASSEMIA
When two carriers become parents, there is a one-in-four chance that any child they have will inherit a thalassemia gene from each parent and have a severe form of the disease. There is a two-in-four chance that the child will inherit one of each kind of gene and become a carrier like its parents; and a one-in-four chance that the child will inherit two normal genes from its parents and be completely free of the disease or carrier state. These odds are the same for each pregnancy when both parents are carriers. If both parents are carriers, then diagnostic tests may be performed on the fetus to see if it is affected.
If both parents are carriers of thalassemia, the fetus will have thalassemia in 1/4 of cases. This can be tested for prenatally using chorionic villus sampling (CVS) or amniocentesis. Early diagnosis is important so that treatment can prevent as many complications as possible.
CYSTIC FIBROSIS
The American College of Obstetricians and Gynecologists (ACOG) recommends that screening tests be available to all couples who are planning pregnancy or who are pregnant. . Testing identifies the most common mutations associated with CF. A genetic mutation is carried by 1 in 29 Caucasians compared to 1 in 46 for those of Hispanic background, 1 in 65 for African Americans, and 1 in 90 for Asian Americans. Both the mother and father will carry a CF mutation in about 1 of 800 pregnancies among Caucasians. Only if both parents are carries is the fetus at risk for CF, and this will occur in 1/4 of cases when both parents are carriers. So, even though a genetic defect is relatively common among Caucasians, CF affects only about 1 in 3200 Caucasian pregnancies.
Some screening protocols test the mother first and the test the father only if the mother carries one of the known mutations, whereas other screening protocols test both the mother and father at the same time. A simple saliva test is often used as the screening test.
Amniocentesis or CVS can be used to identify whether a fetus is affected. However, parents should know this is not performed as part of
a standard genetic amniocentesis, but can be performed in families known to be at increased risk. Following birth, the sweat chloride test is a standard test..
FRAGILE X SYNDROME
About one in 250 women in this country carries the fragile X gene. A woman is considered a carrier of fragile X syndrome if she has either a pre-mutation (60 to 200 trinucleotide repeats within her FMR-1 gene) or a full mutation (greater than 200 trinucleotide repeats within her FMR-1 gene). This can be determined by a blood test. Approximately one-third to one-half of female carriers of a full mutation will exhibit signs of fragile X syndrome, but are a full usually less severely affected than males with full mutations. but are usually less severely affected than males with full mutations. a full mutation will exhibit signs of fragile X syndrome, but are usually less severely affected than males with full mutations.
Amniocentesis or CVS can be used to identify the mutation. However, parents should know this is not performed as part of a standard genetic amniocentesis, but rather only in families known to be at increased risk. Also, a positive test cannot always determine whether a baby will be mentally retarded. While almost all boys who inherit the full mutation have mental retardation or serious learning disabilities, only about one-third to one-half of affected girls will have mental retardation or will have serious learning disabilities, only about one-third to
one-half of affected girls will have mental retardation or serious learning disabilities.
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