Pregnancy can't occur without a complex interplay of hormones that keep the early embryo from being rejected from the body. Progesterone and human chorionic gonadotropin, or hCG, both play crucial roles in early normal pregnancy development. Human chorionic gonadotropin is the hormone measured by serum and urine pregnancy tests, while progesterone helps maintain the pregnancy. Progesterone and hCG support one another so that a pregnancy will continue.
Progesterone levels rise before hCG levels in a normal menstrual cycle that results in pregnancy. After ovulation, the corpus luteum, the leftover shell of the follicle that produced the egg, produces progesterone. Progesterone levels after ovulation normally range from over 10ng/ml to 28 ng/ml even before pregnancy occurs, according to the American Pregnancy Association.
Human Chorionic Gonadotropin Production
An embryo first implants in the uterine wall by 14 days after fertilization. Within 24 to 48 hours, small amounts of hCG are produced by the trophoblast, the layer of the embryo that eventually develops into the placenta. By the first missed period, a urine or serum pregnancy test, which measures the amount of hcG present will be positive. A positive hCG level measures 5ml/IU, but usually reaches 18ml/IU or higher by day one of the missed period, the American Pregnancy Association reports. Only blood tests give exact measurement of hCG levels; urine tests report only a positive or negative result on the test.
Human chorionic gonadotropin signals the corpus luteum to keep making progesterone, which maintains the pregnancy and allows the placenta to develop for the first eight weeks of pregnancy, Alan Beer, M.D. of the Center of Reproductive Immunology and Genetics states. As the placenta grows, it takes over progesterone production. The placenta also produces Gonadotropin-Releasing Hormone, or GnRH, which stimulates production of hCG. Progesterone and hCG both block cells that would attack the growing embryo as an intruder, says Dr. Beer.
Abnormal Progesterone Levels
If progesterone doesn't rise appropriately, the embryo may not implant properly, and the pregnancy may not grow, which results in pregnancy loss. Why some women have lower than normal levels of progesterone isn't known, but low levels of progesterone can occur in women who can't get pregnant with recurrent miscarriage. Low progesterone levels can be treated with supplemental progesterone started immediately after ovulation.
Progesterone supplements can be given in cream, suppository, injection or pill form, with pills being the least well absorbed, according to Dr. Beer. Progesterone levels continue to rise until throughout pregnancy, rising to 300 ng/ml or higher by the end of pregnancy, the American Pregnancy Association reports.
Abnormal HCG Levels
In a normal early pregnancy, hCG levels double every 48 to 72 hours. If hCG levels don't rise appropriately, the cause is usually a chromosomally abnormal embryo. If the pregnancy doesn't grow, hcG levels will fall. As the placenta and embryo stop growing, progesterone levels will fall. After eight weeks, hcG levels begin to drop and remain at a constant level for the rest of the pregnancy, according to the University of Colorado at Boulder, so a drop at this point does not indicate a problem with the pregnancy.