Pregnancy Tests: How They Work & What to Expect

How do pregnancy tests work?

The hormone first enters a woman’s bloodstream when the fertilized egg implants into the lining of her uterus, sometimes as early as six days after the egg has been fertilized. After that, the levels of HCG increase very rapidly – often doubling every 48 hours for a time. Once the levels are detectable in a woman’s urine, her pregnancy test should show positive.

Eggs take about one week to travel from the ovary to the uterus after ovulation. The egg can be fertilized any time during its travels or once it reaches the uterus. HCG only begins production after the fertilized egg implants. It can be hard to predict exactly when fertilization occurred because sperm cells can live for up to five days inside the woman’s body. For this reason, most home pregnancy tests advise women to wait until they are at least two or three days late for their periods – or approximately 15 to 16 days after ovulation – before they take a pregnancy test.

The obstetrician might recommend a blood test done to determine the exact level of HCG in your system. Sometimes a blood test might be needed if they suspect a problem or are concerned about a possible miscarriage. There are two kinds of blood tests done at an OB’s office. A quantitative test measures the amount of HCG in the blood, while a qualitative test simply indicates if the hormone is present or not.

Women who have irregular menstrual periods may have a more difficult time scheduling an at-home urine pregnancy test. Irregular periods are those that have fewer than 21 days between them or that occur more than 35 days apart. Periods that vary from month to month are also considered irregular. In these cases, women should count 36 days from the start of their last period or four weeks from the last time they had unprotected sex before they try a home pregnancy test.

When should I take a pregnancy test, and how often should I take one?

Even though home pregnancy test kits claim they can tell women if they are pregnant even before they miss their periods, most women should wait a few days to a week after they miss their expected period to take a urine pregnancy test. Many women may ovulate later in their cycle than they think, or the fertilized egg may not implant in their uterus for several days. Even then, it takes several days for the HCG hormone to reach high enough levels for strong positive pregnancy test results.

Of course, every pregnancy is different. A woman may produce more or less HCG in each pregnancy, and so she may have positive tests sooner or later in each one. Getting a negative test result or only a faint positive test result does not mean you are not pregnant. (This is called a “false negative.”) In these cases, you should wait a few days and take another test if you still have not gotten your period.

The earlier you take a home pregnancy test, the less HCG there is in your system. The best time to take a urine test is when the urine – and the HCG level – is most concentrated. For this reason, most tests recommend women test when they first wake up in the morning, and that they wait until at least one day after they have missed a period.

You should take a pregnancy test as soon as practical after a missed period or if you think you might be pregnant, following the guidelines above. The earlier you know you are pregnant, the sooner you can think about your options and get the care you need from an obstetrician.

What is a “false positive” pregnancy test and how does it happen?

A pregnancy test false positive result is one that indicates HCG hormones present in a woman’s system, even though she may not be pregnant. Usually, the presence of HCG hormones means she is pregnant. False positive results are uncommon, but they do happen. One reason for a false positive result is an early miscarriage or when a woman has terminated a pregnancy in the eight weeks before becoming pregnant again.

Molar Pregnancies

The woman may also have what is called a molar pregnancy, also known as a hydatidiform mole. This is a non-cancerous or benign tumor that starts when an egg is fertilized but does not develop normally. Instead, the placenta develops into an abnormal mass of cells and cysts. In some cases, an embryo begins to develop, but it is malformed and cannot survive. In other cases, there is no embryo at all. Molar pregnancies can have significant complications, and they require early treatment by a doctor.

Chemical Pregnancies

Women who have early positive pregnancy test results, but get their period soon after that may have what is called a chemical pregnancy. This is a circumstance where the fertilized egg implanted as usual in the uterus, but only long enough to start producing HCG. After that, the egg stopped developing for some reason. Usually, this type of early miscarriage happens when the fertilized egg had some defect that prevented it from growing normally.

A woman who had a faint positive pregnancy test result for a few days, and then tested completely negative, may have had a very early miscarriage. Some experts say 20 to 30 percent of all pregnancies end in miscarriage, usually early. Most women who have very early miscarriages eventually succeed with pregnancy.

Ectopic Pregnancies

One potentially dangerous cause of a false positive pregnancy test result is an ectopic pregnancy. An ectopic pregnancy as any time the fertilized egg implants somewhere other than the main part of the uterus. Usually, this occurs in one of the fallopian tubes leading from the ovaries to the uterus and is called a tubal pregnancy. Sometimes, however, the egg implants in the abdominal cavity, the ovary itself or even the neck of the uterus or the cervix.

No ectopic pregnancy can proceed normally because the fertilized egg cannot survive. In fact, the growing tissue can destroy internal tissues of the mother leading to serious consequences including life-threatening blood loss. Early treatment of an ectopic pregnancy is essential to protect the life of the woman as well as her chances for future healthy pregnancies.

Other Causes of False Positive Pregnancy Tests

Other potential causes of false positive pregnancy tests may include:

• Taking fertility drugs to induce ovulation. These drugs may contain HCG hormone.
• Certain rare medical conditions, such as HCG-secreting tumors.
• Using expired or defective pregnancy testing kits.

Things that should not affect the outcome of a home pregnancy test include most other medications (including prescription and over-the-counter drugs, antibiotics and birth control pills) and alcohol. Women who are taking medicine that contains HCG should see her doctor to get pregnancy tests.