Screening for Down's syndrome: changes in marker levels and detection rates between first and second trimesters

Area of Science:

• Biochemistry
• Genetics
• Maternal-Fetal Medicine

Background:

• Down's syndrome screening relies on biochemical markers in maternal serum.
• First and second trimester screening protocols utilize different marker combinations and gestational timing.

Purpose of the Study:

• To evaluate changes in alpha-fetoprotein (AFP), free beta human chorionic gonadotrophin (F beta hCG), and pregnancy-associated plasma protein-A (PAPP-A) levels in Down's syndrome pregnancies across gestation.
• To compare the accuracy of first-trimester risk assessment with second-trimester screening for Down's syndrome.

Main Methods:

• Maternal serum samples were collected from 47 Down's syndrome pregnancies in both the first and second trimesters.
• Marker levels were compared to control groups to develop and assess identification criteria between trimesters.

Main Results:

• Free beta hCG (F beta hCG) proved effective in both trimesters; PAPP-A was significantly reduced in the first trimester only for trisomy 21.
• First-trimester screening with F beta hCG, PAPP-A, and maternal age achieved a 55% detection rate with a 5% false positive rate.
• First-trimester screening misclassified 12 cases identified by second-trimester screening, indicating a net loss in detection.

Conclusions:

• First-trimester detection rates for Down's syndrome using F beta hCG and PAPP-A may be lower than current second-trimester screening methods.
• Gestation significantly influences the effectiveness of biochemical markers for Down's syndrome screening.