Unit conversion of progesterone: 1ng/mL is roughly equivalent to 3nmol/ml. Serum progesterone levels rise steadily with increasing gestation during pregnancy. The ovarian corpus luteum produces progesterone during the sixth week of pregnancy, and progesterone is mainly secreted by the placenta during mid- and late pregnancy. Progesterone usually acts on the basis of estrogenic effects, for example, it can transform the endometrium into a secretory phase, which is conducive to embryo implantation, while keeping the uterus in a quiescent state before delivery and preventing miscarriage due to uterine contractions. Progesterone also promotes mammary gland follicular secretion in preparation for lactation. Progesterone levels fluctuate with the menstrual cycle during non-pregnancy, with lower progesterone levels during the follicular phase, rising after ovulation, and then gradually decreasing to follicular phase levels before menstruation.