Maternal serum AFP levels in the first trimester of pregnancy.

Maternal serum alphafetoprotein was assayed in 436 women presenting for termination of pregnancy at 6 to 12 weeks. The normal range for each week is presented. There was a progressive rise of AFP levels from week 7 onwards, but a clear separation from non-pregnant values was only apparent after 8 weeks. The definition of a 'low normal' range for the identification of cases at risk of Down's syndrome will probably only be possible at 11 weeks or later.