Detecting and Dating a Pregnancy
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 enlarged and tender breasts, nausea with occasional vomiting, a need to urinate frequently, unusual fatigue, and changes in appetite.
When a menstrual period is late, a woman may wish to use a home pregnancy test to determine whether she is pregnant. Home pregnancy tests detect human chorionic gonadotropin (HCG) in the urine. Human chorionic gonadotropin is a hormone produced by the placenta. 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 performed too early (before the next menstrual period is expected to start). If results are positive, the woman should contact her doctor, who may perform another pregnancy test to confirm the results.
Doctors test a sample of blood or urine from the woman to determine whether she is pregnant. These tests are very accurate. One of these tests, called an enzyme-linked immunosorbent assay (ELISA), can quickly and easily detect a low level of human chorionic gonadotropin in the urine. Some tests can detect the very low level that is present about 1½ weeks 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. Measurement of these levels during the pregnancy can be used to determine whether the pregnancy is progressing normally.
After pregnancy is confirmed, the doctor asks the woman when her last menstrual period was. Pregnancies are conventionally dated in weeks, starting from the first day of the last menstrual period. 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. 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 3 weeks before and 2 weeks after the calculated date is considered normal.
Ovulation usually occurs 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. They are called the 1st trimester (0 to 12 weeks of pregnancy), 2nd trimester (13 to 24 weeks), and 3rd trimester (25 weeks to delivery).
If a woman and her doctor cannot confidently calculate when she became pregnant based on her menstrual period, ultrasonography may be performed to measure the fetus and thus establish the date. For the most accurate measurements, ultrasonography is performed during the first 12 weeks of a pregnancy. An accurate date helps doctors determine whether the pregnancy is progressing normally.
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