If all goes well, four chambers and valves will form by the ninth or 10th week of pregnancy, and the heart will continue developing throughout gestation. But a heartbeat’s familiar “lub-dub, lub-dub” sound is created by the closing of the heart’s valves, which do not exist in the six-week-old cardiac tube.

To opponents of abortion, that is a distinction without a difference. “It is a heart tube, but it is still a heart,” Dr. Francis said. “The shape is different, but that doesn’t change the essence of what it is,” she added…

The consensus among most medical experts is that the electrical activity picked up on an ultrasound at six weeks is not the sound of a heart beating and does not guarantee a live birth. The sound expectant mothers hear during a scan is created by the machine itself, which translates the waves of electrical activity into something audible.

Doctors are partly to blame for the confusion. Many physicians whose patients are excited about a desired pregnancy will use the word “heartbeat” to describe the cardiac activity heard on an early ultrasound. The word has even crept into the medical literature.