(A) Human chorionic gonadotropin is the biochemical basis for pregnancy tests. It is produced by the placenta to help maintain the corpus luteum. Its levels climb rapidly following conception, peaking at about 8 weeks and then gradually decreasing to low levels after 16 weeks. (B) Estrogen does steadily rise throughout pregnancy, increasing to 30 times that of prepregnancy levels. Although estrogen levels do change during pregnancy, it is not used as the main hormone of evaluation in pregnancy tests. (C) α-Fetoprotein is the major protein in the serum of theembryo. It is initially produced by the yolk sac. (D) Lecithin and sphingomyelin are two phospholipids of which fetal lung surfactant is composed. Levels are evaluated to determine fetal lung maturity.