Multiple marker screening for Down syndrome in twin pregnancies

Area of Science:

• Maternal-Fetal Medicine
• Prenatal Diagnostics
• Biochemical Screening

Background:

• Interpreting second-trimester Down syndrome risk in twin pregnancies presents unique challenges.
• Published protocols for Down syndrome screening using maternal serum biochemical markers require validation in twin gestations.
• Maternal age is a known factor associated with increased twinning rates.

Purpose of the Study:

• To assess the real-world performance of a published protocol for interpreting second-trimester Down syndrome risk in twin pregnancies.
• To compare screening performance between twin and singleton pregnancies.
• To estimate the detection and false-positive rates for Down syndrome in twin pregnancies.

Main Methods:

• Retrospective analysis of a cohort of 35,150 pregnancies, identifying 410 twin pregnancies.
• Application of a published maternal serum biochemical marker protocol for Down syndrome risk assessment.
• Statistical comparison of screen-positive rates between twin and singleton pregnancies, adjusting for maternal age and dating method.

Main Results:

• Of 274 twin pregnancies known pre-screening, 15 (5.5%) were screen-positive for Down syndrome.
• Screen-positive rates did not significantly differ between twin and singleton pregnancies when adjusted for covariates.
• Modeled detection rates were 73% for monozygotic and 43% for dizygotic twins with Down syndrome at a 5% false-positive rate, yielding an overall 53% detection rate.

Conclusions:

• The published screening protocol demonstrates practical applicability in twin pregnancies, with comparable false-positive rates to singletons.
• Detection rates for Down syndrome in twin pregnancies are likely lower, particularly for dizygotic twins.
• Laboratories offering multiple marker screening should consider these performance differences when counseling women with twin pregnancies.