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Prenatal Genetic Screening – Serum and Ultrasound
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13. ACOG Resources*
Neural tube defects.
Summary of Recommendations
The following recommendations are based on good and consistent scientific evidence (Level A):
- Periconceptional folic acid supplementation is recommended because it has been shown to reduce the occurrence and recurrence of NTDs.
- For low-risk women, folic acid supplementation of 400 µg per day currently is recommended because nutritional sources alone are insufficient. Higher levels of supplementation should not be achieved by taking excess multivitamins because of the risk of vitamin A toxicity.
- For women at high risk of NTDs or who have had a previous pregnancy with an NTD, folic acid supplementation of 4 mg per day is recommended.
- Maternal serum alpha-fetoprotein evaluation is an effective screening test for NTDs and should be offered to all pregnant women.
- Women with elevated serum AFP levels should have a specialized ultrasound examination to further assess the risk of NTDs.
- The fetus with an NTD should be delivered at a facility that has personnel capable of handling all aspects of neonatal complications.
- The ideal dose for folic acid supplementation has not been appropriately evaluated in prospective clinical studies. A 400 µg supplement currently is recommended for women capable of becoming pregnant.
- The route of delivery for the fetus with an NTD should be individualized because data are lacking that any one route provides a superior outcome.
Neural tube defects. ACOG Practice Bulletin No. 44. American College of Obstetricians and Gynecologists. Obstet Gynecol 2003; 102:203–13.
Prenatal diagnosis of fetal chromosomal abnormalities
Summary of Recommendations
The following recommendation is based on good and consistent scientific evidence (Level A):
- Early amniocentesis (<13 weeks) is not recommended because of the higher risk of pregnancy loss and complications compared with traditional amniocentesis (15–17 weeks).
The following recommendations are based primarily on consensus and expert opinion (Level C):
- Women with singleton pregnancies who will be age 35 years or older at delivery should be offered prenatal diagnosis for fetal aneuploidy.
- Patients with a risk of fetal aneuploidy high enough to justify an invasive diagnostic procedure include women with a previous pregnancy complicated by an autosomal trisomy or sex chromosome aneuploidy, a major fetal structural defect identified by ultrasonography, either parent with a chromosome translocation, and carriers of a pericentric chromosome inversion or parental aneuploidy.
- A combination of one major or two or more minor ultrasound markers of Down syndrome substantially increases risk and warrants further counseling regarding invasive testing.
- The use of ultrasonographic screening for Down syndrome in high-risk women (eg, women age 35 years and older) to avoid invasive testing should be limited to specialized centers.
- With an isolated choroid plexus cyst, testing is indicated only if serum screening results are abnormal or the patient will be older than 32 years at delivery.
- Cervical infections with chlamydia or herpes are contraindications to transcervical CVS.
- Counseling for amniocentesis in a twin pregnancy in women age 33 years is indicated because the midtrimester risk of fetal Down syndrome is approximately the same as for that of a singleton pregnancy at age 35 years.
- Nondirective counseling before genetic amniocentesis does not require a patient to commit to pregnancy termination if the result is abnormal.
Prenatal diagnosis of fetal chromosomal abnormalities. ACOG Practice Bulletin. NUMBER 27, Clinical Management Guidelines for Obstetrician-Gynecologists. Obstet Gynecol. 2001 May;97(5 Pt 1):suppl 1-12.
First-Trimester Screening for Fetal Aneuploidy
ABSTRACT: First-trimester screening for chromosomal abnormalities offers potential advantages over second-trimester screening. Studies in the 1990s demonstrated an association between chromosomal abnormalities and the ultrasonographic finding of abnormally increased nuchal translucency (an echo-free area at the back of the fetal neck) between 10 and 14 weeks of gestation. First-trimester screening using nuchal translucency, free ß-hCG, and pregnancy-associated plasma protein-A has comparable detection rates and positive screening rates for Down syndrome as second-trimester screening using 4 serum markers (alpha-fetoprotein, ß-hCG, unconjugated estriol, and inhibin-A). Although first-trimester screening for Down syndrome and trisomy 18 is an option, it should be offered only if certain criteria can be met.
First-trimester screening for fetal aneuploidy. ACOG Committee Opinion No. 296. American College of Obstetricians and Gynecologists. Obstet Gynecol 2004;104:215–17.
Screening for canavan disease
The Committee on Genetics makes the following recommendations:
- Ideally, molecular carrier screening for Canavan disease should be offered preconceptionally if both members of the couple are of Ashkenazi Jewish genetic background. This screening could be combined with screening for Tay-Sachs disease because both disorders are more common in this group. Many specialized laboratories already offer screening for both diseases. Those with a family history consistent with Canavan disease also should be offered screening, which should be voluntary; informed consent and assurance of confidentiality are required. If potential carriers have not been screened preconceptionally, screening may be offered during early pregnancy.
- If only one partner is of high risk (of Ashkenazi Jewish descent or with a family history consistent with Canavan disease), this partner should be screened first. Ideally, this should be performed preconceptionally. If it is determined that the high-risk partner is a carrier, the other partner should be offered screening. The couple, however, must be informed of the limitations of testing. If the woman is already pregnant, it may be necessary to screen both partners simultaneously so that results are obtained in a timely fashion and to ensure that all options are available for the couple.
- If it is determined by DNA-based analysis that both partners are carriers of Canavan disease, prenatal diagnosis should be offered either by CVS or amniocentesis, using DNA-based testing of the fetal cells.
ACOG committee opinion. Screening for canavan disease. Number 212, November 1998. Committee on Genetics. American College of Obstetricians and Gynecologists. Int J Gynaecol Obstet. 1999 Apr;65(1):91 -2.
Prenatal and preconceptional carrier screening for genetic diseases in individuals of Eastern European Jewish descent
Certain autosomal recessive disease conditions are more prevalent in individuals of Eastern European Jewish (Ashkenazi) descent. Previously, the American College of Obstetricians and Gynecologists recommended that individuals of Eastern European Jewish ancestry be offered carrier screening for Tay-Sachs disease, Canavan disease, and cystic fibrosis as part of routine obstetric care. Based on the criteria used to justify offering carrier screening for Tay-Sachs disease, Canavan disease, and cystic fibrosis, the American College of Obstetricians and Gynecologists' Committee on Genetics recommends that couples of Ashkenazi Jewish ancestry also should be offered carrier screening for familial dysautonomia. Individuals of Ashkenazi Jewish descent may inquire about the availability of carrier screening for other disorders. Carrier screening is available for mucolipidosis IV Niemann-Pick disease type A, Fanconi anemia group C, Bloom syndrome, and Gaucher's disease.
Prenatal and preconceptional carrier screening for genetic diseases in individuals of Eastern European Jewish descent. ACOG Committee Opinion No. 298. American College of Obstetricians and Gynecologists. Obstet Gynecol 2004;104:425–8.
* ACOG Evidence grading system
The MEDLINE database, the Cochrane Library, and ACOG's own internal resources and documents were used to conduct a literature search to locate relevant articles published between January 1985 and October 2000. The search was restricted to articles published in the English language. Priority was given to articles reporting results of original research, although review articles and commentaries also were consulted. Abstracts of research presented at symposia and scientific conferences were not considered adequate for inclusion in this document.
Guidelines published by organizations or institutions such as the National Institutes of Health and the American College of Obstetricians and Gynecologists were reviewed, and additional studies were located by reviewing bibliographies of identified articles. When reliable research was not available, expert opinions from obstetrician–gynecologists were used. Studies were reviewed and evaluated for quality according to the method outlined by the U.S. Preventive Services Task Force:
- I Evidence: obtained from at least one properly designed randomized controlled trial.
- II -1 Evidence obtained from well-designed controlled trials without randomization.
- II -2 Evidence obtained from well-designed cohort or case–control analytic studies, preferably from more than one center or research group.
- I I-3 Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled experiments also could be regarded as this type of evidence.
- III Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.
Based on the highest level of evidence found in the data, recommendations are provided and graded according to the following categories:
- Level A—Recommendations are based on good and consistent scientific evidence.
- Level B—Recommendations are based on limited or inconsistent scientific evidence.
- Level C—Recommendations are based primarily on consensus and expert opinion.
