The presence of human chorionic gonadotropin, or hCG, a hormone produced by the growing placenta during pregnancy, is the basis for both urine and blood pregnancy tests. HCG first appears in your urine within a day of implantation, but at very low levels. The level of hCG in the urine and blood increases daily until between eight and 11 weeks of pregnancy and then drops, the American Pregnancy Association reports.
Significance
The presence of hCG in your urine nearly always signifies pregnancy. A few drugs, such as anticonvulsants, may possibly cause a false positive on a urine pregnancy, MedlinePlus states. If you took an hCG injection for any reason during your menstrual cycle, as is common during an in vitro fertilization cycle, hCG can remain in your system for up to14 days after an injection of 10,000 units of hCG, Baby Hopes reports, giving a false positive on the pregnancy test. After a miscarriage or ectopic pregnancy, hCG levels will remain elevated for up to 35 days, Lab Tests Online states.
Function
The function of hCG is to support the corpus luteum, the fragment of the follicle that produced an egg. The corpus luteum produces progesterone for the first two weeks after ovulation, to support a possibly implanting embryo. The growing placental tissue produces hCG to signal the corpus luteum to keep creating progesterone, and hCG then shows up in the urine.
Timing
Urine pregnancy tests can turn positive as early as 10 to 11 days after conception, but if the embryo implanted a few days later than normal, the test may not be positive for several more days. Most embryos implant between day eight and day 10 after ovulation, Allen Wilcox, M.D. reports in the June 10,1999 issue of "The New England Journal of Medicine." Most pregnancy tests on the market today can detect urinary hCG levels below 100 milli-International units per millilter. A missed period normally ranges from 5 to over 400 mIU/ml, according to the American Pregnancy Association; many tests give accurate results around the time of a missed period.
Measurement
Urine measurements in the urine are qualitative, not quantitative, meaning they don't give a specific reading of the exact amount of hCG in the urine, the American Pregnancy Association explains. They record only whether hCG is present in sufficient quantities to give a positive reading or not.
Considerations
Most women want to know whether they're pregnant as soon as possible. Urine pregnancy tests give very early answers to that question, in most cases. While false positives are rare, false negatives are common, and occur most often because the test is done too soon to detect sufficient levels of hCG. Using the first urine sample of the morning gives the best results, because the sample is more concentrated and more likely to contain an adequate amount of hCG if you're pregnant, Lab Tests Online advises. If you get a negative reading but your period doesn't come, test again in a few days; as many as 10 percent of women have late implantation of the embryo, Pregnancy.org states.
References
- "The New England Journal of Medicine"; Time of Implantation of the Conceptus and Loss of Pregnancy; Allen Wilcox, M.D.; June 10, 1999
- Lab Tests Online:HCG
- American Pregnancy Association: Human Chorionic Gonadotropin
- Pregnancy.org: Home Pregnancy Tests
- Baby Hopes: If I Had an HCG Shot,When Can I Take A Pregnancy Test And Get Accurate Results?
