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HCG is found in your urine or blood around 10 to 11 days after conception (when a sperm fertilizes an egg). Your hCG levels are the highest towards the end of the first trimester (10 weeks of pregnancy), then decline for the rest of your pregnancy.  Human chorionic gonadotropin is a hormone produced primarily by syncytiotrophoblastic cells of the placenta during pregnancy. The hormone stimulates the corpus luteum to produce progesterone to maintain the pregnancy. Smaller amounts of hCG are also produced in the pituitary gland, the liver, and the colon.  Most experts theorize that the reason for changes in hCG levels is that, at a certain point in the pregnancy, the placenta takes over making the hormones estrogen and progesterone. This means hCG is no longer needed to stimulate the ovary to make hormones, says Dr. Ross.  During pregnancy, you make high levels of estriol, a type of estrogen hormone. This hormone keeps you uterus healthy and helps it grow along with the developing baby. Estriol levels peak toward the end of pregnancy to get your body ready for childbirth and breastfeeding.  Estriol is produced by the placenta by the conversion of 16-hydroxy-dehydroepiandrosterone sulfate to androgens, which are subsequently aromatized to estriol.