Estradiol, often abbreviated as E2, is the most biologically active of the estrogens and is secreted in women mainly by the ovaries. It promotes and maintains the growth of female reproductive organs, breast and long bones and plays an important role in the maintenance of female sexual characteristics. It is also produced in large quantities by the placenta in women after the third trimester of pregnancy. In men, it is mainly produced by the adrenal cortex and testes and plays an important role in protein, lipid, water-electrolyte and calcium-phosphorus metabolism. Reference values of estradiol in normal women during the menstrual cycle: Follicular phase: early 79.7±3.8pg/ml; late 144±10.3pg/ml Ovulation: 214.1±27pg/ml Luteal phase: early 122.7±8.2pg/ml; mid 184±7.0pg/ml; late 106±8.4pg/ml Menopause: 6.3±4.4pg/ml Normal male: 25.5±7.8pg/ml The clinical significance of estradiol measurement is as follows: (1) Before puberty, if estradiol levels are elevated, it helps in the diagnosis of precocious puberty in females. (2) During the menstrual cycle, dynamic observation of estradiol levels can assist in judging and determining the timing of ovulation. Before ovulation estradiol tends to be around 360 pg/ml, while on the day of peak LH, estradiol decreases and reaches its lowest point the following day. (3) During normal pregnancy, blood levels of estradiol increase with the pregnancy cycle, and the increase is more pronounced in twin and multiple pregnancies.