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History and Physical Exam
Embryologists date the development of the embryo and fetus from the time of conception ("embryonic age"). In contrast, clinicians conventionally date pregnancy from the first day of the last menstrual period ("gestational age").
The last menstrual period (LMP) system of gestational dating is based on an "ideal" 28-day cycle, with fertilization occurring on day 14. Dating by LMP is most reliable if the patient consistently records normal monthly cycles interrupted by the sudden cessation of menses. However, only about 15% of reproductive-aged women have the idealized 28-day cycle,10 and approximately 25% of pregnant women bleed during the first trimester.11
The reliability of historical dating is further compromised by the fact that fertilization can occur from 6 days before to 3 days after the presumed date of ovulation.10
History and Physical Exam
Investigators have examined the value of historical factors in the diagnosis of pregnancy among women who request hCG testing. The sensitivity of reported signs and symptoms such as breast tenderness, missed menses, amenorrhea, and morning sickness was in the range of 30% to 70%.12,13
Physical signs suggestive of early pregnancy include changes in the breasts and Hegar's sign, a softening of the cervical isthmus occurring around 6 weeks' gestation. Like historical factors, physical signs during the first trimester are unreliable predictors of pregnancy. Although fairly specific, physical signs lack sensitivity.13 A low sensitivity means that most women who are pregnant lack these signs (a high rate of false negatives).
Enlargement of the uterus begins shortly after implantation and, after about 4 weeks' gestation, the rate of enlargement is approximately 1 cm per week. As a result, clinicians can perform a bimanual exam to generate a rough estimate of gestational age.
A study of medical abortion with mifepristone/misoprostol in China, Cuba, and India found that dating based on LMP closely correlated with estimates based on physical exam.14 However, a number of extenuating factors, such as obesity, extreme uterine retroversion, and fibroids, may compromise accurate uterine sizing in early pregnancy.
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Implantation begins approximately 5 to 7 days after fertilization. At this time, trophoblastic cells begin to produce human chorionic gonadotropin (hCG). Serum levels of this hormone rise linearly during the first 6 weeks of pregnancy. During this early stage of pregnancy, hCG levels double every 1.3 to 2 days.15-21
Human chorionic gonadotropin in urine is extremely stable and concentrations are nearly identical to those in blood. As a result, measurement of urine hCG serves as an excellent marker for the detection of early pregnancy. Modern urine pregnancy tests using monoclonal antibodies to the ß subunit of hCG can detect hCG concentrations as low as 25 mIU/mL. By 7 days after implantation - about the time of the missed menses - these tests are positive in 98% of women.22
Radioimmunoassay techniques for measuring concentrations of serum β-hCG can detect levels as low as 2 to 10 mIU/mL. Results from quantitative tests may differ among laboratories, so whenever possible, serial pregnancy tests should be obtained from the same laboratory.23
During early pregnancy, serum ß-hCG levels correlate roughly with gestational age. Pregnant women with levels under 5,000 mIU/mL are unlikely to be over 42 days' gestation.24,25 Therefore, when ultrasonography is not available, measurement of ß-hCG may offer a potentially useful means of determining the gestational age criterion for eligibility for medical abortion.
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Proceed to Ultrasound Imaging in Early Pregnancy.
References for this module