Detection of hCG in maternal blood and urine is the basis for endocrine assays of pregnancy. Syncytiotrophoblasts produce hCG in amounts that increase exponentially during the first trimester following implantation. With a sensitive test, the hormone can be detected in maternal serum or urine by 8 to 9 days after ovulation. The doubling time of serum hCG concentration is 1.4 to 2.0 days. (Ref: William's Obstetrics; 25th edition)