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Detecting and Dating a Pregnancy

By Haywood L. Brown, MD, F. Bayard Carter Professor of Obstetrics and Gynecology, Duke University Medical Center

Pregnancy begins when an egg is fertilized by a sperm. For about 9 months, a pregnant woman’s body provides a protective, nourishing environment in which the fertilized egg can develop into a fetus. Pregnancy ends at delivery, when a baby is born.

If a menstrual period is a week or more late in a woman who usually has regular menstrual periods, she may be pregnant. Sometimes a woman may guess she is pregnant because she has typical symptoms. They include the following:

  • Enlarged and tender breasts

  • Nausea with occasional vomiting

  • A need to urinate frequently

  • Unusual fatigue

  • Changes in appetite

Home pregnancy tests

When a menstrual period is late, a woman may use a home pregnancy test to determine whether she is pregnant. Home pregnancy tests detect human chorionic gonadotropin (hCG) in urine. Human chorionic gonadotropin is a hormone produced by the placenta early in pregnancy.

Results of home pregnancy tests are accurate about 97% of the time. If results are negative but the woman still suspects she is pregnant, she should repeat the home pregnancy test a few days later. The first test may have been done too early (before the next menstrual period is expected to start). If results are positive, the woman should contact her doctor, who may do another pregnancy test to confirm the results.

Did You Know...

  • Results of home pregnancy tests are accurate about 97% of the time.

Laboratory pregnancy tests

Doctors test a sample of urine or sometimes blood from the woman to determine whether she is pregnant. These tests are more than 99% accurate.

One of these tests, called an enzyme-linked immunosorbent assay (ELISA), can quickly and easily detect even a low level of human chorionic gonadotropin in urine. Some tests can detect the very low level that is present several days after fertilization (before a menstrual period is missed). Results may be available in about half an hour.

During the first 60 days of a normal pregnancy with one fetus, the level of human chorionic gonadotropin in the blood approximately doubles about every 2 days. These levels can be measured during the pregnancy to determine whether the pregnancy is progressing normally.

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