By the time you are about six weeks pregnant, your baby’s heartbeat will already be visible on ultrasound. Then, at around 10 weeks, a Doppler ultrasound will allow you to hear your baby's heartbeat.
The first time you see or hear your baby’s heartbeat is a joyous and exciting moment–and also very reassuring as it is a sign that the pregnancy is going well and that the fetus is developing.
When you first hear your baby's heartbeat, you may notice that it sounds very fast, but don’t worry because that is completely normal. At six weeks, your baby's heart beats at about 100 to 120 beats per minute, speeding up to about 160 to 180 beats per minute at 10 weeks, then slowing to 130 to160 beats per minute by the beginning of your second trimester.
Dr. Peter Doubilet and Dr. Carol B. Benson are Harvard Medical School professors of Radiology. They teach and see patients at the Brigham and Women's Hospital in Boston and are co-authors of Your Developing Baby (McGraw-Hill, 2008), a pregnancy guidebook that takes readers through every stage of a baby's development, using 2D and 3D ultrasound images, original explanatory diagrams and reader-friendly prose. Yourdevelopingbaby.com