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Biparietal diameter
5%
1/21
Femur length
10%
2/21
Abdominal circumference
Serum beta-hCG
19%
4/21
Crown-rump length
57%
12/21
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This patient presents for her first prenatal visit and is likely in her first trimester based on the timing of her last menstrual period. The most accurate method of dating a pregnancy in the first trimester is ultrasound measurement of the crown-rump length. Early gestations develop at consistent rates with minimal variability in the timing of certain milestones such as presence of fetal cardiac activity and measurements such as crown-rump length. This latter measurement is the standard method of dating early pregnancies and is defined as the length between the top of the fetal head and the bottom of the buttocks. As the gestation progresses, there is increasing variability among fetal growth, and discrepancies in measurements should not change the previously established dating. Instead, they are used to identify abnormalities such as growth restriction or presumed macrosomia. Illustration A is an ultrasound showing the crown-rump length which is the distance between the top of the fetal head and the bottom of the buttocks. Incorrect Answers: Answer 1: The biparietal diameter is measured at the level of the thalami of the brain and is part of the Hadlock growth scan, which is used in the second or third trimester for determining fetal growth. It is not used early in pregnancy due to inability to accurately identify specific brain structures. Answer 2: Femur length is part of the Hadlock growth scan but is not used in the first trimester as fetal bone calcification is incomplete. Answer 3: Abdominal circumference is part of the Hadlock growth scan but is not used in the first trimester. Answer 4: Serum beta-hCG levels vary widely, and though there are general ranges for specific periods of pregnancy, they are not precise enough to be used for fetal dating. In addition, serum beta-hCG levels fluctuate during the course of pregnancy and do not follow a linear trend that could be used for dating. Bullet Summary: In the first trimester of pregnancy, measurement of the crown-rump length on ultrasound is the most precise method of dating.
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