A viral Facebook post claims that “A fetus at 6 weeks has no more brain than a grain of rice.” The post references the Texas heartbeat abortion law, which prohibits abortion when a fetal heartbeat can be detected, usually at six weeks.

The first claim in the post—that a baby during the sixth week of gestation is the size of a grain of rice—is accurate. Gestational calendars use the beginning of a woman’s last cycle as a start date, but conception usually occurs two weeks later, meaning that at this point, the embryo has...