The Supreme Court recently upheld a Texas law that would be prevent patients from accessing abortion care after six weeks of pregnancy. There are many reasons this law is concerning—chiefly that it will do considerable harm to many people—but it is also based on bad biology. Pregnancy math is confusing, and it’s unclear whether legislators involved are simply ignorant on reproductive biology or recognize that it’s an indirect way to ban all abortions.

But in reality, the six-week ban limits abortion care to only four weeks after conception, and only one week, realistically, from when a person could find out they are pregnant. At this stage, an embryo has implanted and has a neural tube, and the blood vessel that will develop into the heart begins pulsing. This pulsing, or “heartbeat,” is the basis for the emotional appeal of these bills. But at this early stage, the embryo is still in the process of differentiating organs and won’t be classified as a “fetus” until about a month later.

The reason pregnancy math works so strangely is practical: it’s easier to pinpoint the first day of someone’s last period and count from that point as a standard marker because dates of ovulation and conception are harder to identify. But counting pregnancy as beginning during the last period includes two weeks prior to actual pregnancy and can inspire public health policy considering all women of reproductive age to be “pre-pregnant,” such as health messages that recommend that all women of reproductive age abstain from alcohol. Given a lack of adequate education in health and biology—educational information that is often another target for evangelical Christians—some may think “six weeks of pregnancy” is plenty of time to realize you are pregnant. But at only four weeks post-conception, and three weeks post-implantation, there is a limited window to even affirm pregnancy.

This is where pregnancy math meets menstrual math, which is further complicated by the limits of hormonally detecting pregnancy. Menstrual math, or predicting when a “missed” period occurs, is often based on an assumed 28-day cycle. If you have a regular 28-day cycle, the expected missed period should happen two weeks after conception. That gives you about two weeks before that “six-week” threshold to take a pregnancy test and see your doctor. But it’s recommended that you wait for a week after your missed period to take a pregnancy test, because if you take it too early, you may get a false negative. Pregnancy tests measure human chorionic gonadatropin (hCG), a hormone produced after implantation. Though it can be potentially detected shortly after implantation, at about a week after conception and “three weeks” pregnant, it may not build up to detectable levels until a couple of weeks later. Thus, for patients with a predictable 28-day cycle, there is only about one week before the “six-week” threshold to confirm pregnancy. For someone who knows they want an abortion, taking a test, getting confirmation from a health care provider and having the abortion would have to occur within a single week.

But most people who menstruate do not have a regular 28-cycle. That neat, four-week cycle is a simplification of a more variable process.  One study found about 13 percent of women have that “typical” 28-day cycle. The average is close to it, but it varies within a range of about 21 to 40 days and may not be consistent from month-to-month. A person with an average cycle length of 29 days may have a cycle that is 27 days one month, 29 another, and 31 another. If their period hasn’t arrived “on time,” they may not notice until it’s been a week late.

Imagine another person with a 35-day cycle. Ovulation and conception may not even occur until “three weeks of pregnancy.” The earliest they may be able to detect a pregnancy is at “four to five weeks” pregnant. They won’t be expecting their period until “five weeks” and may not even remark it as a late period until “six weeks.” By the time they have taken a pregnancy test, they have only been carrying that embryo in their body for three weeks, and they have already missed the window to access abortion. Such variability in cycle may be even more common for adolescents, perimenopausal woman, transgender men receiving gender-affirming hormonal treatments, and people undergoing other health crises or significant stressors.

There are many reasons someone may want an abortion, and often detection of pregnancy happens after that six-week threshold. This Twitter thread provides heartbreaking examples, including abuse victims, children as young as 11, trans men grappling with dysphoria and women with wanted pregnancies that were not viable. Ultimately, these decisions should be between pregnant people and their doctors, not politicians. In the Catholic tradition I was raised in, I was taught that the soul begins at conception—but also that dogs do not have souls. Both of those theological positions are now recognized as less certain by Pope Francis. American evangelical opposition to abortion is tightly entangled with white supremacist ideas about outreproducing other racial and religious groups. Everyone should have the right to follow their own religious convictions—but legislators’ personal beliefs should not dictate medical decisions between a pregnant person and their health care provider.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.