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].
* Calculated risks are based on midtrimester risk of chromosome
singletons as provided by the California Department of Health Services Genetic Disease
1. Matias A,et al .Down syndrome screening in multiple pregnancies.
Obstet Gynecol Clin North Am. 2005;32(1):81-96
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.