Abstract
Daily measurements of serum oestradiol, progesterone, Schwangerschaftspro-tein 1 (SP1) and hCG were made in 13 menstrual cycles during which conception occurred. The assays were continued until 21 d after ovulation. SP1 was detected in maternal blood 8–14 d after ovulation and hCG 9–13 d after ovulation. We conclude that these two proteins are secreted simultaneously but independently by the syncytiotrophoblast.