This test measures the concentration of progesterone in the blood. Progesterone is a hormone whose main role is to help prepare and maintain a woman’s endometerium (lining of the womb/uterus) for pregnancy; it works together with several other female hormones.On a monthly basis, the uterus and ovaries undergo a cyclical change under the control of hormones. The first half of the cycle is characterised by the hormone oestrogen, produced by the ovaries, causing the endometrium to grow and replenish itself.Increasing oestrogen levels cause a surge in luteinising hormone (LH) mid-cycle leading to the release of an egg from one of the two ovaries. In the second half of the cycle the corpus luteum (a small yellow mass of cells) forms in the ovary at the site where the egg was released and begins to produce progesterone. This progesterone stops endometrial growth and prepares the uterus for the possible implantation of a fertilised egg. If fertilisation does not occur, the corpus luteum degenerates, progesterone concentrations fall, and the endometrial lining is shed (menstruation).Progesterone levels (produced from the corpus luteum) peak 7 days prior to the onset of menses. If a menstrual cycles is regular and conforms to a standard 28 day length, progesterone concentrations will normally peak on approximately day 21 of the cycle (with day 1 being the first day of menstruation). Therefore, when assessing for ovulation, progesterone should be measured at day 21 of the cycle or 7 days prior to the expected onset of menses. If a woman has an irregular or prolonged cycle then progesterone can be measured on a weekly basis until the onset of menses.If an egg is fertilised following ovulation, and becomes implanted in the uterus, the corpus luteum continues to produce progesterone. After several weeks, the placenta replaces the corpus luteum as the main source of progesterone, producing relatively large amounts of the hormone throughout the rest of a normal pregnancy.
