First trimester combined screening - focus on early biochemistry.

First trimester combined screening (cFTS) for foetal trisomy 21 has become an established method in many countries. The screening is based on a combination of maternal-age-related risk, ultrasound (nuchal translucency) and two maternal serum biochemical markers, free beta human chorionic gonadotropin (FbhCG) and pregnancy associated plasma protein A (PAPP-A). The concentrations of these biochemical markers are affected by several maternal and pregnancy factors, which are discussed herein. Improvements in the algorithm have extended the screening to include trisomy 21 in mono- and dichorionic twin pregnancies, trisomy 18, trisomy 13 and triploidy. The results from large databases have shown that the screening algorithms are efficient for a range of rare autosomal trisomies and marker chromosomes and for a broad range of other chromosomal aberrations. Recent data show that the strength of the individual markers is highly dependent on the gestational age of sampling and indicate a general increase in the performance of the screening for trisomy 21 when using blood samples from early in the first trimester at gestational age 8-10 weeks.