Pregnancy tests

A pregnancy test detects human pregnancy hormone (human chorionic gonadotropin (hCG)) to determine whether an individual is pregnant.

HCG testing can be performed on a blood sample or on urine.

hCG rises quickly in the first few weeks of pregnancy, peaking at 10 weeks.

hCH is produced by the syncytiotrophoblast cells of the fertilized ova as the cells invade the uterus’ lining and start forming what will become placenta.

Urine tests will typically show positive around four weeks after the last menstrual period.

A urine pregnancy test alone is not sufficient to exclude pregnancy.

A urine pregnancy test sensitivity is dependent on when the last active intercourse occurred, the ovaulatory cycle phase, and urine concentration.

Tests are best done in the morning as hCG levels are then highest.

Urine hCG test can detect concentrations ranging from 12-50 mLU/liter.

Sensitivity of urine pregnancy test is 90% at the time of a missed period, but only 40% in the week prior.

A urine pregnancy test can remain positive up to four weeks after delivery, miscarriage, or abortion.

A positive result is less likely to be incorrect than a negative one.

How much water/fluids have been consumed can affect the results.

Blood hCG tests for a more specific part of the hCG molecule and can detect pregnancy earlier than urine.

Blood hCG tests can detect pregnancy even before a period has been missed.

The order of detection from earliest to latest is that hCG can be detected earliest in the blood, then a urine test, then ultrasound.

Tests for pregnancy look for the presence of the beta subunit of human chorionic gonadotropin (hCG) in the blood or urine.

The thresholds for a positive test are generally determined by an hCG cut-off where at least 95% of pregnant women would get a positive result on the day of their first missed period.

hCG can be detected in urine or blood after implantation around six to twelve days after fertilization.

Some evidence suggests that hCG may be released preimplantation as well.

Quantitative blood serum beta tests can detect hCG levels as low as 1 mIU/mL.

Typically a positive pregnancy test is at 5mIU/mL.

Urine test strips have detection thresholds of 10 mIU/mL to 100 mIU/mL, depending on the brand.

Obstetric ultrasonography of the gestational sac sometimes can be visualized as early as four and a half weeks of gestation approximately two and a half weeks after ovulation, and the yolk sac at about five weeks’ gestation.

With US the embryo can be observed and measured by about five and a half weeks.

The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks’ gestation.

Early pregnancy factor (EPF) can be detected in blood within 48 hours of fertilization, rather than after implantation.

Homebpregnancy test kits, when used by experienced technicians, are almost as accurate as professional laboratory testing-97.4%.

When used by consumers, however, the accuracy falls to 75%.

False negative readings can occur when testing is done too early.

False negative results can occur occurs when there is excess hCG in the urine saturating the detection antibody and prevents formation of the necessary antibody -antigen-antibody sandwich effects and can occur with gestational trophoblastic disease when pathologically high levels of intact hCG are present.

Quantitative blood tests and the most sensitive urine tests usually begin to detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation.

As hCG levels continue to rise through the first 20 weeks of pregnancy, the chances of false negative test results diminish with time.

At four weeks gestation measured from the first day of the last menstrual period, urine hCG typically ranges from 6-750 international units per liter, although it may be as high as 19,950 mL units per mL.

Less sensitive tests may not detect pregnancy until three or four days after implantation.

In the urine, an hCG degradation product known as beya-core fragment is the major form of hCG during the late first entire second trimester.

Point of care urine hCG tests have a sensitivity of 90%-97% during the first week after a missed menstrual period, and a specificity of 99.2%

Because menstruation occurs on average 14 days after ovulation, the likelihood of a false negative is low once a menstrual period is late.

The accuracy of a pregnancy test is most closely related to the day of ovulation.

Sperm may live up to five days in the fallopian tubes, waiting for ovulation to occur, and it could take up to 12 further days for implantation to occur.

The most sensitive pregnancy tests may give false negatives up to 17 days after pregnancy.

In some home pregnancy tests high hCG detection thresholds of up to 100 mIU/mL), may take an additional three or four days for hCG to rise to detectable levels.

In these tests false negatives may occur up to three weeks after the act of intercourse or insemination that causes pregnancy.

False positive test results may occur for several reasons: including errors of testing, use of drugs containing the hCG molecule, and non-pregnant production of the hCG molecule.

Urine tests can be falsely positive in women taking medications: chlorpromazine, phenothiazines and methadone among others.

hCG injection as part of infertility treatment will result in a positive pregnancy tests that assay hCG, regardless of the actual pregnancy status.

Certain cancers-liver and other medical conditions may produce elevated hCG and thus cause a false positive pregnancy tests and include choriocarcinoma and other germ cell tumors, IgA deficiencies, heterophile antibodies,, gestational trophoblastic diseases.

Pregnancy testing is used to determine the viability of a pregnancy by serial quantitative blood tests, usually 3–4 days apart.

Below an hCG level of 1,200 mIU/ml the hCG usually doubles every 48–72 hours, though a rise of 50–60% is still considered normal.

Between 1,200 and 6,000 mIU/ml serum the hCG usually takes 72–90 hours to double, and above 6,000 mIU/ml, the hCG often takes more than four days to double.

The failure of hCG to increase normally may indicate an increased risk of miscarriage or a possible ectopic pregnancy.

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