Lawmakers in nine US states have passed laws banning abortions when a fetal heartbeat is detected or within 6 weeks of pregnancy, according to data from the Guttmacher Institute, a non-profit organization.

But, what exactly do we mean when we talk about “fetal heartbeat” during six weeks of pregnancy? Although some people may imagine a heart-shaped organ beating inside a fetus, it is something else.

At week six of pregnancy, an ultrasound can detect “a little flutter in the area that will become a future baby’s heart,” said Dr. Saima Aftab, medical director of the Fetal Care Center at Nicklaus Children’s Hospital in Miami.

The flutter occurs because the group of cells that will become the heart’s future “pacemaker” gain the ability to fire electrical signals, she says.

While the heart is far from fully formed at this point, the “pulse” is not audible; Aftab told Live Science that if doctors put a stethoscope into a woman’s abdomen early in pregnancy, they wouldn’t hear a heartbeat.

Only in the past few decades have doctors been able to detect this flutter in six weeks, thanks to the use of more sophisticated ultrasound techniques, Aftab said. Previously, the technology wasn’t advanced enough to detect a flutter that occurs early in pregnancy.

She added that although the detection of this flutter appears important, it “in no way translates into the viability of the heart” or the viability of the pregnancy.

The heart still has a lot of development before it can be fully formed. In fact, the first trimester of pregnancy is the period of “organ formation,” Aftab says.

She said that after detecting the flutter at six weeks, the heart muscle continues to develop during the next four to six weeks, and undergoes the folding and bending process that must occur for the heart to take its final shape.