A 26-year-old woman, gravida 1, para 0, comes to the physician for a prenatal visit. She did not keep accurate records of her menstrual periods but the last one was approximately 7 months ago. Early in the pregnancy, an ultrasound showed that she was at approximately 6 weeks gestation, giving her a current gestational age of 32 weeks. The patient's blood pressure is 110/60 mm Hg and pulse is 90/min. On examination, her fundal height is 26 cm. Fetal heart tones by Doppler shows a rate of 140/min. An ultrasound is performed and calculates the patient's gestational age as 28 weeks based on the fetal biparietal diameter and femur length. The measured fetal abdominal circumference is consistent with a gestational age of 26 weeks. Which of the following is the most accurate method for estimating the current gestational age?

Question

A 26-year-old woman, gravida 1, para 0, comes to the physician for a prenatal visit.  She did not keep accurate records of her menstrual periods but the last one was approximately 7 months ago.  Early in the pregnancy, an ultrasound showed that she was at approximately 6 weeks gestation, giving her a current gestational age of 32 weeks.  The patient's blood pressure is 110/60 mm Hg and pulse is 90/min.  On examination, her fundal height is 26 cm.  Fetal heart tones by Doppler shows a rate of 140/min.  An ultrasound is performed and calculates the patient's gestational age as 28 weeks based on the fetal biparietal diameter and femur length.  The measured fetal abdominal circumference is consistent with a gestational age of 26 weeks.  Which of the following is the most accurate method for estimating the current gestational age?

Quizplus · Accepted Answer

11ec25c8_da75_25b6_bc81_cba7c1ce071f_MD0003_00 Ultrasound is commonly used to estimate the gestational age of pregnancy. First trimester ultrasound with crown-rump length measurement is the most accurate method of determining gestational age. It becomes less accurate as the pregnancy progresses as there is minimal variability in size among fetuses during the first trimester. The accuracy varies from +/- 3 to 5 days between 7-14 weeks gestation. If an ultrasound in the second or third trimesters shows a discrepancy between estimated gestational age and fetal measurements (seen in this patient), growth problems should be considered (eg, fetal growth restriction, macrosomia). The estimated gestational age (EGA) from the first trimester ultrasound should not be changed as it is more accurate than ultrasound estimates in the second and third trimesters. (Choices A, B, and F) After the first trimester, fetal abdominal circumference, biparietal diameter, femur length, and head circumference are used to estimate gestational age. The accuracy in the second trimester ranges from +/-1-2 weeks and decreases to +/- 2-3 weeks in the third trimester. The estimated fetal weight (EFW) can also be determined from these measurements and varies in accuracy by +/- 15%. (Choice D) After 20 weeks gestation, fundal height can be measured in centimeters. This is done by measuring from the pubic symphysis to the top of the fundus. This measurement correlates with the gestational age by +/- 3 weeks. However, fundal height measurement can be confounded by leiomyomata and obesity; it is therefore less accurate than first trimester ultrasound dating. fundal height (Choice E) Last menstrual period (LMP) is commonly used for EGA, but may also be inaccurate. Pregnancy dating by LMP assumes a normal 28-day cycle with fertilization occurring on day 14. Many women do not have a 28-day cycle and fertilization may occur before or after day 14. In a patient with a reliable LMP and normal menses, the estimated date of delivery and gestational age are based on LMP. If the EGA varies by >7 days in the first trimester and >10 days in the second trimester, then ultrasound EGA is used rather than LMP. Educational objective: Ultrasound dating with fetal crown-rump measurement in the first trimester is the most accurate way to determine estimated gestational age (EGA). EGA should not be changed based on measurement discrepancies on a second or third trimester ultrasound. References: Ultrasonography in pregnancy. (http://www.ncbi.nlm.nih.gov/pubmed/19155920)

A 26-Year-Old Woman, Gravida 1, Para 0, Comes to the Physician