Risk of Chromosome Abnormality in Multiple Gestation

The calculator below calculates the theoretical risk for Down syndrome and chromosome abnormalities in multiple gestations.* At least for twin gestations, the theoretical risk may be an overestimate of the observed risk of Down syndrome in some populations [2,3].

Maternal Age (or age of eggs )

Down Syndrome

All chromosomal aneuploidies

One
affected One or both affected Both
affected One
affected One or
both affected Both
affected

Twin
One placenta 1 in
1 in

Twin
Two placentas 1 in
1 in
1 in
1 in

Twin
Unknown # placentas 1 in
1 in

Triplet
Three placentas
1 in

Quadruplets
Four placentas
1 in

REFERENCES

* Calculated risks are based on midtrimester risk of chromosome abnormalities for
singletons as provided by the California Department of Health Services Genetic Disease Branch

1. Matias A,et al .Down syndrome screening in multiple pregnancies. Obstet Gynecol Clin North Am. 2005;32(1):81-96 PUBMED
2. Jamar M, et al. A low rate of trisomy 21 in twin-pregnancies: a cytogenetics retrospective study of 278 cases. Genet Couns. 2003;14:395-400.PUBMED
3. Cuckle H. Down's syndrome screening in twins.J Med Screen. 1998;5(1):3-4.PUBMED

All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgement. 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.