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Outcome of First-Trimester Pregnancies with Slow Embryonic Heart Rate at 6–7 Weeks Gestation and Normal Heart Rate by 8 Weeks at US¹

¹ From the Department of Radiology, Brigham and Women's Hospital and Harvard Medical School, 75 Francis St, Boston, MA 02115. Received May 18, 2004; revision requested August 3; revision received August 24; accepted October 1. Address correspondence to P.M.D. (e-mail: Pdoubilet{at}Partners.org).

PURPOSE: To determine retrospectively the outcome of pregnancies in which the embryo has a slow heart rate at 6.0–7.0 weeks gestation and a normal heart rate at follow-up ultrasonography (US) by 8.0 weeks gestation.

MATERIALS AND METHODS: Institutional review board approval was granted; informed consent was not required. The study was Health Insurance Portability and Accountability Act compliant. Singleton pregnancies with an embryonic heart rate measured on a 6.0–7.0-week US scan were identified. The heart rate was classified as slow if it was fewer than 90 beats per minute prior to 6.3 weeks or fewer than 110 beats/min at 6.3–7.0 weeks, normal if it was 100 or more beats/min at less than 6.3 weeks or 120 or more beats/min at 6.3–7.0 weeks, or borderline if it was 90–99 beats/min prior to 6.3 weeks or 110–119 beats/min at 6.3–7.0 weeks. Pregnancies were excluded from the analysis if they were lost to follow-up before the end of first trimester. The Fisher exact test was used for all statistical comparisons.

RESULTS: The rates of first-trimester demise were 60.6% for pregnancies with slow heart rates at 6.0–7.0 weeks (188 of 310), 17.4% for those with borderline heart rates (103 of 593), and 9.1% for those with normal heart rates (186 of 2034). There were 59 pregnancies with a slow heart rate at 6.0–7.0 weeks and a normal heart rate at follow-up US by 8.0 weeks; 15 (25.4%) resulted in first-trimester demise. This rate of demise was significantly higher than that of 7.2% (28 of 390) in pregnancies with a normal heart rate at 6.0–7.0 weeks and a normal heart rate by 8.0 weeks (P <.001, Fisher exact test). Pregnancies with a borderline heart rate early in pregnancy followed by a normal heart rate had a demise rate of 7.6% (nine of 118), which is similar to those with normal heart rates early in pregnancy followed by normal heart rates at follow-up (P = .84).

CONCLUSION: When a slow embryonic heart rate is detected at 6.0–7.0 weeks, the likelihood of subsequent first-trimester demise remains elevated (approximately 25%) even if the heart rate is normal at follow-up. In such pregnancies, at least one follow-up scan in late first trimester is warranted.

© RSNA, 2005