Congressional Republicans have introduced a federal heartbeat bill, a controversial measure that would ban abortion as soon as a fetal heartbeat can be detected, about six weeks into the pregnancy.

U.S. Rep. Steve King (R-Iowa), who introduced the bill last week, did not respond to a request for comment about why fetal heartbeats are at the center of the proposed law.

Even setting aside the serious reproductive health implications of such a ban, bills that use a “fetal heartbeat” as any kind of benchmark are based on a flawed understanding of science. Such bills are of a piece with the dismayingly widespread scientific illiteracy among legislators. (Take, for instance, the RNC platforms on pornography as a health problem and support of abstinence-only sex education, two stances that are not rooted in scientific evidence; this is to say nothing of many policymakers’ dangerously wrongheaded attitudes toward climate change.)

“There is no scientific basis for prohibiting a woman from obtaining an abortion whether there is or isn’t a heartbeat present,” Dr. David Eisenberg, a board-certified OB-GYN and abortion provider in St. Louis, told The Huffington Post.

‘Heartbeat’ bills fundamentally misunderstand fetal development

When you’re talking about a fetus, the term “heartbeat” isn’t even really accurate. A fetal heartbeat is typically detectable at around five or six weeks into a pregnancy, according to Dr. Rebecca Cohen, an assistant professor of obstetrics and gynecology at the University of Colorado.

“It’s not a fully formed heart like you would understand from looking at an adult or even a young child,” Cohen said. “It’s a very early structure. We can see it on the ultrasound, but it’s not a heart, a fully developed organ, by any means.”

Moreover, cardiac activity isn’t a credible measure of fetal viability. This preliminary activity starts at a point in the pregnancy where there’s still a significant risk of miscarriage, and no real guarantee that the pregnancy will continue to be a healthy, uncomplicated one.

“Even with seeing that cardiac activity, about 5 to 10 percent of pregnancies will go on to miscarry, because there are other abnormalities that we can’t detect on the ultrasound at those early stages,” Cohen said.

‘Heartbeat’ bills create an extremely narrow abortion window

Then there’s the fact that the six-week window works against women who aren’t actively planning pregnancy, who might not be carefully tracking their menstrual cycles, or who have irregular cycles. There’s also such a thing as it being too early in a pregnancy to have an abortion.

In the very early stages after conception, it’s difficult to tell whether a pregnancy will grow normally, or whether it’s even located in the uterus. Ectopic pregnancies, where the fertilized egg implants somewhere other than the uterus, occur in about 20 out of every 1,000 pregnancies, according to the Mayo Clinic.

Doctors use transvaginal ultrasounds to determine whether a pregnancy is ectopic, but that diagnostic method isn’t necessarily reliable until four or five weeks into a pregnancy. Ectopic pregnancies may also require surgery, so it’s important that doctors are certain about the diagnosis before they terminate a pregnancy.

“It’s actually a very, very narrow window,” Cohen said. “It can’t be too early, it can’t be too late, now with these potential bills.”

The American Congress of Obstetricians and Gynecologists opposed King’s legislation, telling HuffPost: “H.R. 490 places politicians squarely between America’s women and their physicians. We stand strong against turning back the clock on women’s health.”

Restrictive abortion bills have a history of bad science

In December, Ohio Gov. John Kasich (R) declined to sign a six-week fetal heartbeat abortion ban in favor of a 20-week abortion ban, which more than a dozen other states already have in place.

“It’s politically clever,” Cohen said. “By introducing those two bills at the same time, the 20-week ban doesn’t look so bad.”

But it is. For starters, only about 1 percent of abortions take place after 20 weeks, and those are nearly always due to a previously undetectable fetal abnormality. In practice, this means the women most affected by 20-week bans tend to be those carrying fetuses with severe birth defects.

Financial and logistical burdens aside, 20-week abortion bans don’t make much sense from a medical perspective.

Twenty-week bans are based on the idea that fetuses can feel pain at 20 weeks’ gestation, a supposition that’s scientifically murky at best. Doctors and researchers know very little about pain in adults, and discussions about fetal pain tend to rely on studies of fetal response to stimuli, which isn’t a reliable proxy for pain. Such discussions often devolve into theoretical arguments.

Then there’s the fact that fetuses rarely survive outside the womb at 20 weeks. That means 20-week bans are in tension with the Supreme Court’s Roe v. Wade decision, which protects a woman’s right to have an abortion until the fetus has a reasonable chance of surviving outside the womb.

Limiting abortion access leads to worse medical outcomes

Cohen warned that it’s a mistake to think limiting women’s access to abortion actually reduces the number of abortions performed.

“Those limits don’t prevent women from accessing abortion,” she said. “They only prevent women from accessing safe abortion.”

A total of 21 million unsafe abortions were performed in 2008, accounting for 13 percent of maternal deaths worldwide, according to the World Health Organization. Those unsafe abortions can involve self-inflicted bodily trauma, ingesting chemicals or dangerous drugs, or relying on people without formal training to perform abortions, sometimes in unhygienic conditions.

Lacking access to better treatment, some women order the abortion pill from illegal, unreliable or disreputable sources. In the U.S., women sometimes cross the border to purchase the drugs in Mexico.

“Although the medication regimens themselves are extremely safe, if you’re ordering it online, you can’t guarantee you’re actually getting what you think you’re getting,” Cohen said.

Unfortunately, women are pushed into such unsafe health decisions in part because of bills that have little basis in science.

“I have been involved in the provision of abortion care for over 15 years and can say that there is no rational, scientific, medical or public health reason that would make sense to prohibit women from obtaining the care they need,” Eisenberg said. “Bans such as these will not reduce the number of abortions, but will result in more women being hurt.”

CORRECTION: An earlier version of this article stated that there were 21 million deaths from unsafe abortions in 2008. In fact, there were 21 million unsafe abortions and 47,000 deaths.