First trimester fetal cardiac activity. Sonographic documentation of a progressive early rise in heart rate.

The heart rates of 124 first trimester fetuses were determined with real-time sonography and were analyzed with regard to gestational age. The mean embryonic heart rate increased from 101 beats per minute (bpm) at 5 to 5.95 menstrual weeks to 143 bpm at 8 to 8.95 weeks. After nine weeks, the rate reached a plateau, ranging from 137 to 144 bpm. Slower heart rates are normal early in the first trimester and should not be misinterpreted as an abnormal finding or mistaken for maternal pulsations.