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Maternal Age and Gestational Age-Specific Aneuploidy Risk

Maternal Age and Gestational Age-Specific Risk for Chromosomal Abnormalities BETA

This calculator estimates theoretical baseline risk before incorporating cfDNA/NIPT, serum screening, ultrasound markers, or diagnostic testing. Trisomy 21 is estimated using the updated Snijders 1999 gestational-age model; trisomy 18, trisomy 13, Turner syndrome, and triploidy use the broader Snijders 1995 chromosomal-defect tables. Turner syndrome is the only sex chromosome abnormality modeled as a selectable risk estimate; other sex chromosome aneuploidies are discussed below but are not individualized by this calculator.

Clinical use: Use as a pretest-risk estimate for counseling. A positive cfDNA screen should be followed by genetic counseling, comprehensive ultrasound, and the opportunity for diagnostic testing. A negative cfDNA screen lowers risk for the targeted conditions but does not exclude all chromosomal or genetic disorders.
T21: 20–45 years; T18/T13: 20–44 years
Trisomy table range: 10–40 weeks
47,XXX, 47,XXY, and 47,XYY are not modeled as individualized estimates.

Results

Enter maternal age and gestational age, then select Calculate.

Detailed linked references
  1. Snijders RJM, Sebire NJ, Nicolaides KH. Maternal age and gestational age-specific risk for chromosomal defects. Fetal Diagn Ther. 1995;10(6):356-367. Also available from Karger.
  2. Cuckle HS, Wald NJ, Thompson SG. Estimating a woman's risk of having a pregnancy associated with Down's syndrome using her age and serum alpha-fetoprotein level. Br J Obstet Gynaecol. 1987;94(5):387-402. doi: 10.1111/j.1471-0528.1987.tb03115.x.
  3. Snijders RJM, Sundberg K, Holzgreve W, Henry G, Nicolaides KH. Maternal age- and gestation-specific risk for trisomy 21. Ultrasound Obstet Gynecol. 1999;13(3):167-170. doi: 10.1046/j.1469-0705.1999.13030167.x.
  4. Hook EB, Cross PK, Schreinemachers DM. Chromosomal abnormality rates at amniocentesis and in live-born infants. JAMA. 1983;249(15):2034-2038.
  5. Matias A, Gomes C, Flack N, Montenegro N, Nicolaides KH. Screening for chromosomal abnormalities at 10–14 weeks: the role of ductus venosus blood flow. Ultrasound Obstet Gynecol. 1998;12(6):380-384. doi: 10.1046/j.1469-0705.1998.12060380.x.
  6. Norton ME, Jacobsson B, Swamy GK, et al. Cell-free DNA analysis for noninvasive examination of trisomy. N Engl J Med. 2015;372(17):1589-1597. doi: 10.1056/NEJMoa1407349.
  7. American College of Obstetricians and Gynecologists. Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol. 2020;136(4):e48-e69.
  8. Society for Maternal-Fetal Medicine. Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester. Am J Obstet Gynecol. 2021;225(4):B2-B15.

Reference notes: Trisomy 21 uses the updated Snijders 1999 model/table. Trisomy 18, trisomy 13, Turner syndrome, and triploidy use the broader Snijders 1995 chromosomal-defect tables. Turner syndrome is included as the sex chromosome abnormality modeled by gestational age; other sex chromosome aneuploidies reported by Snijders are not individualized in this calculator. Cuckle and JAMA/Hook data are included as historical maternal-age baseline-risk sources. Matias is included because abnormal ductus venosus was an important first-trimester ultrasound-marker literature source, but it is not used as a calculator multiplier in this baseline-risk page.

Sex chromosome aneuploidy note

Snijders et al. included Turner syndrome (45,X) as a gestational-age-specific prevalence estimate and assumed that its prevalence was not related to maternal age. Therefore, Turner syndrome is included in this calculator, but it is not maternal-age adjusted.

Other sex chromosome aneuploidies, including 47,XXX, 47,XXY, and 47,XYY, were reported in Snijders et al. as observed frequencies in live-birth and prenatal cohorts. They are not presented here as individualized maternal-age and gestational-age-specific calculator outputs because Snijders did not provide the same detailed risk-table model used for trisomy 21, trisomy 18, trisomy 13, Turner syndrome, and triploidy.

Clinical interpretation: cfDNA screening for sex chromosome aneuploidy has different performance characteristics than screening for the common autosomal trisomies. Positive sex chromosome aneuploidy screens should be interpreted with genetic counseling, ultrasound context, and the option of diagnostic testing.

Disclaimer

All calculations must be independently confirmed before clinical use. These estimates are intended for educational and counseling support and are not a substitute for clinical judgment, genetic counseling, or diagnostic testing when indicated.