Calculator

The calculator below applies Snijders maternal age- and gestational-age singleton baseline risks to simplified multiple-gestation probability models. For twins, the theoretical risk may overestimate observed Down syndrome rates in some populations (see references).

Enter whole years (20-44)
Enter whole weeks (10- 40)
Output format: "1 in N" | Theoretical pre-cfDNA estimates; confirm and interpret clinically.

Pregnancy type Down syndrome Combined trisomy 21/18/13
One affected One or both affected Both affected One affected One or both affected Both affected
Twin - one placenta - - - -
Twin - two placentas - -
Twin - unknown # placentas - - - -
Triplet - three placentas - - - - -
Quadruplets - four placentas - - - - -
Probability assumptions used for the "theoretical risk"

This calculator is labeled theoretical because it applies simplified probability models to a baseline singleton maternal age- and gestational-age risk from Snijders et al. tables 2–4.

  • Dichorionic / "multiple placentas" pregnancies (modeled as independent fetuses): assumes each fetus has the same baseline probability p (from the selected singleton Snijders baseline) and that fetal outcomes are independent.
    Typical relationships (twins): P(both) = p˛, P(exactly one) = 2p(1−p), P(one or both) = 1 − (1−p)˛. (Triplets/quadruplets extend similarly.)
  • Monochorionic / "one placenta" twins (modeled as monozygotic): assumes the pregnancy is effectively "all or none" for a chromosomal aneuploidy (i.e., both fetuses affected or neither). Under that simplifying assumption, the "one or both" risk is the same as "both affected."
    Real-world exceptions exist (e.g., mosaicism, diagnostic classification issues), so observed data may differ.
  • Unknown placentation: uses the historical approximation retained from the original page (approximately 5/3 × the singleton fetal risk) to provide a single “one or both affected” estimate.
  • Known limitation: published observations suggest the theoretical Down syndrome risk in twins may overestimate observed rates in some populations (see Matias, Jamar, and Cuckle references).

Display note: "combined trisomy 21/18/13" is calculated as the sum of the Snijders singleton probabilities for trisomies 21, 18, and 13 at the selected maternal age and gestational age; it is not an "all chromosome abnormalities" estimate.

Disclaimer: All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither Perinatology.com nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.

References

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* Baseline singleton risks on this page use the Snijders tables for trisomies 21, 18, and 13 by maternal age and gestational age. Multiple-gestation outputs are theoretical probability estimates and should be used only as pretest counseling estimates when cfDNA screening and diagnostic testing results are not available.

  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. 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. PubMed
  3. Matias A, Montenegro N, Blickstein I. Down syndrome screening in multiple pregnancies. Obstet Gynecol Clin North Am. 2005;32(1):81-96, ix. doi:10.1016/j.ogc.2004.10.001. PubMed
  4. Jamar M, Lemarchal C, Lemaire V, Koulischer L, Bours V. A low rate of trisomy 21 in twin-pregnancies: a cytogenetics retrospective study of 278 cases. Genet Couns. 2003;14(4):395-400. PubMed
  5. Cuckle H. Down's syndrome screening in twins. J Med Screen. 1998;5(1):3-4. PubMed
Notes for use
  • These outputs are intended for education and counseling support, not as standalone clinical decision-making.
  • Interpret in context of chorionicity, method of conception, screening approach, and local observed rates.
  • Use primarily when cfDNA screening or diagnostic testing is absent; once screening results are available, post-test risk should be recalculated with test-specific performance or reported PPV.
  • Confirm any risk statement with patient-specific context and local screening protocols.