How Pregnancy Tests Work

In the first few weeks after conception, you may notice changes in your body and in the way you feel. A late menstrual period is usually the first pregnancy symptom. However, many other conditions, from stress to certain chronic illnesses, can delay the onset of menses, so a late period is not always a reliable sign until at least two weeks after the date you expect it.

On the other hand, you can have spotting while pregnant, so the presence of some bleeding doesn't eliminate the possibility of pregnancy.

In the first weeks of pregnancy, you may feel more fatigued than usual. You may experience nausea or vomiting, especially in the morning, a week or two after your missed period. Your breasts may have some tingling or tenderness and may even enlarge. The areolae (the dark areas around the nipples) may darken even more. If you have been having trouble getting pregnant and are recording your basal body temperature, the temperature may remain elevated. But just as with a late period, all of these signs and symptoms, if they occur at all, can have other causes. They do not prove you are pregnant.

If you go to see your doctor when your period is two weeks late, he or she may find physical changes that suggest you are pregnant. Your vagina and cervix may be blue to purplish because of increased blood flow. This is known as the Chadwick sign. The uterus may feel softer, larger, and more round.

Many women know they are pregnant before they see their doctor, however, because they perform a pregnancy test themselves at home. Nonprescription home pregnancy tests are available in any pharmacy and cost $10 to $20. These tests are designed to detect the presence in the urine of human chorionic gonadotropin (HCG), a hormone produced by the placenta shortly after fertilization.

Home pregnancy tests on the market today vary in sensitivity. Some can detect HCG one day after the missed period. Others require one to two weeks. Some tests must be done on a urine specimen obtained in the morning, when the concentration of HCG is the highest; others can be performed on a specimen collected anytime. Some tests react within one minute, but others require one to two hours. If you follow the directions carefully, the results are 90 to 95 percent accurate.

The tests are easy to perform. You immerse a trip of paper into a collected urine sample, or you urinate on a strip of paper. Positive tests usually are indicated by the formation of a lone or a plus sign on the paper. Now, there are even digital tests that display the results in a window so that there is no need to interpret lines or plus signs.

Even though these tests are extremely sensitive and accurate, it is possible to receive a false-positive or false-negative result. If you are taking fertility drugs, they may affect the outcome of the test and produce a false-positive result -- that is, the test result is positive even though the woman is not pregnant. However, more common than a false-positive test result is a false-negative one -- that is, the test result is negative even though the woman is pregnant. This usually occurs when the test is done too early after the missed period.

In some cases, the level of HCG is too low to detect, or the embryo hasn't become implanted yet. In approximately ten percent of women, the embryo implants after the first day of the missed period. Low levels of HCG may also be caused by an ectopic pregnancy (a pregnancy that develops outside the uterus). If the first test result is negative and your period doesn't start, repeat the test in five to ten days. If it is still negative, and you believe you may be pregnant, consult your doctor.

Your doctor will probably order a urine pregnancy test similar to the home pregnancy tests. If your doctor needs to know if you are pregnant at a time too early for the urine test to be used or if he or she suspects a false-negative test result, your doctor may order a blood test that is more sensitive and specific. Because it can measure very small amounts of HCG, your doctor can use it to diagnose pregnancy before a missed period (seven to nine days after fertilization) or to diagnose a tubal pregnancy (one that develops in one of the fallopian tubes). The test takes longer (two to three hours) to complete, and it is more expensive because it requires special equipment and personnel. Therefore, it is not used routinely to diagnose pregnancy.

When you finally know for sure that you are pregnant, your next question will undoubtedly be "When will my baby be born?" Delivery usually occurs 280 days after the first day of the last menstrual period. An easier way to calculate the delivery date, or due date, is to add nine months and seven days to the first day of your last normal menstrual period. Most women don't give birth on the exact date, but 80 percent give birth within ten days of this date -- either ten days before or ten days after.

As your pregnancy progresses, your doctor can double-check your due date with the timing of certain events. For example, the baby's heart is usually heard at 10 to 12 weeks. The level at which the top of the uterus can be palpated (felt) by the doctor is another clue; at 20 to 22 weeks, for example, it is usually at the umbilicus (navel). If your physician performs an ultrasound examination, the technician can measure the baby's head and compare the result with standard tables to estimate gestational age.

Discovering that you are pregnant is a thrilling moment. The next nine months will be filled with excitement both for you and your family.