The Texas law, known as Senate Bill 8, bars abortions once “cardiac activity” can be detected in an embryo. Such activity consists of electrical pulses that can be picked up on ultrasound but are not yet true heartbeats, since heart valves form later in the process of embryonic development.

Cardiac activity is detectable about six weeks after conception. Critics say this law amounts to a near total ban on abortion in part because many women do not realize they are pregnant yet at that point. By the time women miss their periods, they are already about four weeks pregnant, and some women have irregular cycles or do not track their periods carefully enough to know the exact date their last ones started.

The law contains an exception, permitting abortions after cardiac activity is detected in cases of a medical emergency. But it makes no exceptions for cases of rape or incest.

The practical effect of the law has been a sharp drop in legal abortions in Texas — though figures for September do not show as sharp a drop as many experts had predicted.