How soon can a pregnancy be detected?

Modern pregnancy testing is both sophisticated and sensitive. Gone are the days when a woman had to wait at least two weeks after a missed period to determine whether she was pregnant, weeks that were stressful whether she was hoping she was pregnant or praying she was not.

Nowadays ordinary blood tests can accurately diagnose pregnancy as early as one or two days after a missed period. Urine tests conducted in a doctor’s office may be taken a day or so later. Home pregnancy test kits also are quite accurate as little as two days after a missed period.

How do pregnancy tests work?

All pregnancy tests, whether of urine or blood, look for the presence of beta human chorionic gonadotropin, the beta subunit of human chorionic gonadotropin, a hormone produced by the dividing cells of the embryo even before it is implanted within the uterus — though human chorionic gonadotropin levels become detectable only after implantation. The hormone makes its way into the mother’s blood (and thereafter into her urine) through the bloodvessels of the placenta. The tests work by measuring antibodies to this hormone.

There are also special blood tests that use radioactively labeled human chorionic gonadotropin. These tests, called radio immunoassays or radio receptorassays, can detect pregnancy even before you miss a period. Because they are expensive, they are used only in special situations; for example, to check women at high risk for an ectopic pregnancy or those who have some medical condition (such as diabetes or kidney disease) that increases the risks of pregnancy.

Are some tests more accurate than others?

Blood tests are a little more reliable than urine tests, but the urine tests, even the home kits, are quite accurate. The blood tests in general can detect as little as 25 units of human chorionic gonadotropin in the sample; the urine tests do not turn positive until the human chorionic gonadotropin level has risen to something like 50-100 units, which happens a day or so later, since in a normally developing pregnancy the level of human chorionic gonadotropin in the mother’s blood doubles every forty-eight hours or so.

Urine tests and regular blood tests give qualitative results: they tell you whether or not you are pregnant. By repeatedly measuring the amount of human chorionic gonadotropin, more sophisticated quantitative tests can indicate how well the pregnancy is going.

Where can you get a pregnancy test, and how much does it cost?

You can have a urine test at your doctor’s office, a private laboratory, or a clinic. You can buy a home pregnancy test kit at a pharmacy. In some communities, women’s health centers or family planning organizations like Planned Parenthood offer tests free or at minimal cost. Blood tests must be done in laboratories.

Urine tests done in your doctor’s office cost about twenty-five dollars. Qualitative blood tests, which give a simple yes or no answer, are done by a laboratory and cost forty-five to fifty dollars. Your insurance may cover the costs.

How accurate are pregnancy tests?

Modern pregnancy tests are generally reliable, though no test is 100 percent accurate. Occasionally false positives result (indicating that you are pregnant when you are not) or false negatives (indicating that you are not pregnant when you are). Sometimes a test is inconclusive and must be repeated.

False negatives can come about if the test is done too soon in the pregnancy or too late, since human chorionic gonadotropin levels fall again after the second month of pregnancy. Sometimes abnormal pregnancies or pregnancies on the verge of spontaneous abortion will give false negatives. If a urine sample has been contaminated or has sat too long without refrigeration, the test may yield a false negative.

False positives can come about if the test “mistakes” luteinizing hormone for human chorionic gonadotropin, to which it is chemically similar. Luteinizing hormone levels spike at the time of ovulation and are also elevated in the urine of older women approaching menopause. Certain medications can skew the test: tranquilizers, antidepressants, methadone, and drugs for high blood pressure that contain methyldopa. Marijuana and even large quantities of aspirin can produce false positives. After you have a miscarriage or abortion, a pregnancy test will read positive for about ten additional days.

How accurate are home pregnancy testing kits? How expensive?

Home pregnancy tests use the same techniques as the urine tests you take in a doctor’s office. If you follow the directions correctly, they are just as sensitive as the office tests and will detect pregnancy as early as two days after a missed period. They are about 98 percent reliable, but do give occasional false positives and negatives.

They cost between ten and twenty-five dollars, a little less perhaps than the urine tests in a doctor’s office; but you must pay for a home test kit out of pocket, while most insurance companies will pay for pregnancy testing in an office or lab setting.

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