When a couple makes the decision to have a child, waiting to know if the woman is pregnant can be an exciting but stressful time. Home pregnancy tests detect the appearance of a hormone called human chorionic gonadotropin in a woman's urine. HCG is produced by the placenta and appears in the blood shortly after the embryo implants in the uterus, followed a few days later by its appearance in the urine.
A newly conceived embryo travels down a woman's fallopian tube and into her uterus, where it implants into the uterine wall 6 to 12 days after fertilization. Within a few days, membranes surrounding the tiny embryo begin secreting hCG into the mother's blood.
A blood test can detect hCG about 9 to 15 days after fertilization, but it takes a few more days for hCG from the blood to appear in the urine, where it can be measured with a home pregnancy test. In a study published in the April 2005 issue of "Fertility and Sterility," hCG was measured in blood and urine of 63 women in early stages of pregnancy. The hormone appeared in urine samples about 6 days after it could be measured in blood, or by 2 1/2 to 3 weeks after fertilization.
Testing for the presence of hCG in a woman's urine typically involves dipping a detection stick into a cup of collected urine, or a woman holds a stick in the stream of urine as it's released. In either test, a color change, line, symbol or digital message indicates a positive result. Usually, the test is taken 1 or 2 days after a missed period and is about 97 percent accurate, according to the American Pregnancy Association.
Because the time to appearance of hCG in urine can be variable, a negative test doesn't always indicate a failure to conceive. The American Pregnancy Association recommends that a woman who has a negative test but early signs of pregnancy, such as a missed period, nausea or breast tenderness, should repeat the test or see her doctor for a more sensitive test for hCG in her blood.
Predicting the Outcome
Because the detection of hCG in urine varies among women, researchers have asked whether the time it appears might predict the final success of a pregnancy. In a study published in the April 2005 issue of "Fertility and Sterility," the authors followed the study subjects and found there was no difference in the day hCG appeared among women who had spontaneous abortions and those who had successful pregnancies. This suggests that, if hCG appears relatively slowly in a woman's urine, it's unlikely to indicate an early problem with the pregnancy.
Special tests that measure the chemical nature of hCG can be useful in predicting some pregnancy problems. For example, appearance of a type of hormone called hyperglycosylated hCG, along with other markers, can help detect a fetus with Down syndrome.
When hCG is released into a woman's blood, it passes into many organs where it can be changed chemically. For example, the kidneys convert much of the hCG into a form called the beta-core form, so only about 20 percent of the hCG in blood passes unchanged into urine, with the rest being mostly beta-core hCG, which a standard test doesn't detect.
Although not currently available, in the future a test that measures beta-core hCG could be useful in identifying pregnancy in very early stages. A study published in the July 2006 issue of "Biology of Reproduction" studied another type of hCG, called bioactive, in a group of pregnant women. The authors found that bioactive hCG levels that rose steadily through pregnancy were associated with live births. They concluded that measuring this type of hormone could help predict the success of a pregnancy, although more research is needed to confirm this.