One of the oldest descriptions of a home pregnancy test comes from ancient Egypt, where women who suspected they were pregnant would urinate on wheat and barley seeds.
If the wheat grew, they believed, it meant a girl was on the way; the barley sprouting meant a boy. If neither grew, she wasn't pregnant.
But that was way back then, how do home pregnancy tests work today?
When a woman is pregnant, the hormone human chorionic gonadotropin (hCG) is made by cells that nourish the egg after it has been fertilised and becomes attached to the uterine wall, ultimately forming the placenta.
Levels of hCG double every 48-72 hours, reaching their peak around 8-11 weeks after conception, before declining and levelling off for the rest of the pregnancy. The hormone is found in the urine of pregnant women, although it takes a few weeks for hCG to build up to levels that can be detected in a home pregnancy test.
The minimum amount of hCG that can be detected in urine is 6.5 milli-international units per millilitre (miU/ml), but no current over-the-counter tests go this low.
Most home pregnancy tests are very easy to use. Most require that you simply urinate on the stick provided and wait 3-5 minutes for it to show the result.
Some display the words "pregnant" or "not pregnant", others show one line for "not pregnant" and two lines for "pregnant", and some even give an estimate of the number of weeks along.
"We would caution not to put too much stock in these gestation estimators as we often see clients who haven’t been calculated correctly by the tests," says Andrea Thompson, advocacy manager at Marie Stopes South Africa.
Home pregnancy testing strips are also available. One of these is dipped into a container of urine, and the wait is up to 8 minutes. A colour change will normally give the result. Most tests recommend that the first urine of the morning is used for the test.
How early in the pregnancy can a home pregnancy test be used?
The sensitivity of the home pregnancy test will determine how early in the pregnancy the hormone can be detected. The lower the miU/ml measurement of the test, the earlier the pregnancy can be detected.
Most home pregnancy tests have a sensitivity of 20-25 miU/ml, meaning they will be most accurate 12-15 days after implantation, or a day or two after a missed period.
Are home pregnancy tests always accurate?
No, says Johannesburg gynaecologist Dr Kiran Kalian. "Home pregnancy tests can give false positives for several reasons, including the test itself being expired, or hCG being produced in the body for a reason other than pregnancy, such as by the pituitary gland or by a rare form of cancer."
Certain drugs, such as tranquillisers and anticonvulsants, may also cause false-positive results. False negatives (when the test says you're not pregnant but you are) can occur if the test has expired, if you test too early in the pregnancy, or if you have a molar pregnancy (where a foetus doesn't form properly in the womb and a baby doesn’t develop).
If your home pregnancy test is negative but you believe you’re pregnant, wait at least a week before taking another test so that the hCG levels in your urine can accumulate to detectable levels.
How does the blood test differ from the urine test?
A blood test is done by a medical practitioner to test for pregnancy. This test is 100 percent sensitive and specific for pregnancy. Dr Kalian explains why. "hCG in pregnancy comes in two main forms in human blood: regular hCG and hCG-H. Together they make up total hCG."
"Regular hCG comes in two forms, alpha and beta. Urine tests measure total hCG. Blood tests measure the beta unit of regular hCG. A positive result is when the beta-hCG in the blood is greater than 5 miU/ml."
hCG can first be detected in a blood sample as soon as seven days after conception. If I get a positive urine test, should I still have a blood test for pregnancy?
Even the most sensitive home pregnancy tests should be verified by a medical professional. "A woman should make an appointment to see a nurse or a doctor as soon as possible if she thinks she's pregnant so that the nurse or doctor can more accurately calculate how far along the pregnancy is," says Thompson.
"At the same time, she or he can provide more details about the next steps and care to take for a healthy pregnancy."
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