Progesterone is a hormone produced during the early part of the menstrual cycle by the ovaries; after ovulation the remnant of the ovulated follicle, the corpus luteum, produces progesterone. Once a woman gets pregnant, the corpus luteum supports the growing placenta until the placenta begins to produce progesterone at around 10 weeks, according to the American Pregnancy Association. Progesterone helps maintain a pregnancy by decreasing the immune response, which would normally attack a growing fetus and by decreasing uterine smooth muscle contractions that might dislodge an attaching embryo, among other tasks.
Week 1 Progesterone Levels
While it may seem strange to count the first two weeks before ovulation as part of the first trimester of pregnancy, pregnancy is counted as 40 weeks, beginning with the first day of the last menstrual period. So the first two weeks of the menstrual cycle are part of the first trimester of pregnancy. Progesterone levels during the follicular phase of the menstrual cycle, which include follicular growth before ovulation, are generally less than 1 nanogram per milliliter (ng/ml), although normal lab values vary slightly, the U.S. National Library of Medicine site Medline Plus states.
Week 3 Progesterone Levels
After ovulation, progesterone rises as the corpus luteum produces progesterone. The normal range, according to Medline Plus, is 5 to 20 ng/ml in midcycle; progesterone levels should be around 15 ng/ml to sustain a pregnancy, should one occur, Drs. Aniruddha and Anjali Malpani of the Malpani Infertility Clinic state. Progesterone levels peak around day 20 and then begin to drop as the corpus luteum deteriorates if no pregnancy occurs that month. Progesterone changes the lining of the uterus so that the embryo is able to implant; low levels may prevent implantation. If a woman has a history of low progesterone levels and early pregnancy loss of infertility, progesterone supplements may be prescribed to maintain this level. Supplements can be given as injections, pills or vaginal suppositories.
Week 4 to 6 Progesteone Levels
By the time of the first missed period, a pregnancy is measured as four weeks, although the embryo probably implanted only one week before. Progesterone levels at four weeks may vary considerably, but average from 12 to 20 ng/ml from week four to six, according to the American Pregnancy Association. It's important to understand that supplementing a pregnancy with low progesterone and other low pregnancy hormones such as human chorionic gonadotropin and estrogen won't save a pregnancy; usually if all three hormone levels are low, the pregnancy is not viable.
Weeks 7 to 12 Progesterone Levels
Progesterone levels continue to rise throughout the first trimester, and generally range from 9 to 47 ng/ml, the American Pregnancy Association states, and rise 1 to 3 ng/ml a day before reaching a peak for the trimester.
