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Detecting and Dating a Pregnancy

By

Raul Artal-Mittelmark

, MD, Saint Louis University School of Medicine

Last full review/revision Aug 2019| Content last modified Aug 2019
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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.

Due date

Pregnancies are conventionally dated in weeks, starting from the first day of the last menstrual period.

After pregnancy is confirmed, the woman’s doctor asks her when her last menstrual period was. The doctor calculates the approximate date of delivery by counting back 3 calendar months from the first day of the last menstrual period and adding 1 year and 7 days. For example, if the last menstrual period was January 1, the doctor counts back 3 months to October 1, then adds 1 year and 7 days. The calculated due date is October 8 the next year. Only 10% or fewer of pregnant women give birth on the calculated date, but 50% give birth within 1 week, and almost 90% give birth within 2 weeks (before or after the date). Delivery between 2 weeks before and 2 weeks after the calculated date is considered normal.

The ovaries usually release an egg (called ovulation) about 2 weeks after a woman’s menstrual period starts, and fertilization usually occurs shortly after ovulation. Consequently, the embryo is about 2 weeks younger than the number of weeks traditionally assigned to the pregnancy. In other words, a woman who is 4 weeks pregnant is carrying a 2-week-old embryo. If a woman’s periods are irregular, the actual difference may be more or less than 2 weeks.

Pregnancy lasts an average of 266 days (38 weeks) from the date of fertilization (conception) or 280 days (40 weeks) from the first day of the last menstrual period if the woman has regular 28-day periods. Pregnancy is divided into three 3-month periods, based on the date of the last menstrual period:

  • 1st trimester: 0 to 12 weeks of pregnancy

  • 2nd trimester: 13 to 24 weeks

  • 3rd trimester: 25 weeks to delivery

The most accurate way to determine when a baby is due is ultrasonography, particularly if it is done during the first 12 week.

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