Confusion Over How Pregnancy Dates Are Measured Is Widespread

How this confusion also makes for uninformed debates on abortion.

Graphic of person's brain with question marks around it
Pregnancies are dated using the first day of the person's last menstrual period, which is often two to three weeks before conception. Cage Rivera/Rewire News Group illustration

This article is republished from The Conversation under a Creative Commons license. Read the original article, “Confusion over how pregnancy dates are measured is widespread – and makes for uninformed debate over abortion limits.”

By Laura Elder, Mary-Kate Lizotte, and Steven Greene

Most Americans don’t know two key facts about pregnancy, including how they are dated and how long a trimester is—and this could matter, as a growing number of states place restrictions on abortion.

Florida enacted a new law on May 1 that bans abortions after six weeks of pregnancy, with a few exceptions—including documented rape, incest, and to save the life of the pregnant person.

Florida joins the majority of Southern states that now have complete bans or highly restrictive abortion laws, enacted since the Supreme Court overturned the federal right to get an abortion in June 2022. Many of the restrictive laws ban abortion after a set number of weeks.

Anti-abortion rights groups, such as Susan B. Anthony Pro-Life America, have called the six-week abortion ban the gold standard in abortion policy. Florida Republicans supporting the bill have labeled it a reasonable compromise between a full abortion ban and few abortion restrictions.

Some OB-GYNs have explained that many people do not even know they are pregnant at six weeks. Research shows that people on average find out they are pregnant at five and a half weeks. About 23 percent do not know until seven weeks of pregnancy or later.

So, do Americans, including those enacting six-week bans, actually understand how the timing or dating of pregnancy works?

We are scholars of political science, gender and public opinion, and are writing a book about public opinion on abortion after the Supreme Court’s reversal of the federal right to get an abortion in June 2022.

How does pregnancy work?

To gain insights into this issue, we developed a few pregnancy questions and included them in a research survey in late September 2023. The survey had 1,356 respondents, who were broadly representative of the U.S. population. The respondents’ median age was 46. Approximately 49 percent of these people were men, while 70 percent were white and 29 percent were college graduates. Meanwhile, 43 percent of them were Democrats, and 38 percent were Republicans.

The first question asked respondents how pregnancies are dated. The correct answer is that pregnancies are dated using the first day of the person’s last menstrual period, which is often two to three weeks before conception.

The second question asked about trimesters. Many Americans are familiar with the term trimester, and polling consistently shows that Americans find abortion most acceptable during the first trimester. We asked Americans if they knew approximately how many weeks a trimester was. The correct answer is 13.

Americans’ pregnancy knowledge

We found that only one-third of respondents knew how pregnancy is dated. A majority—approximately 60 percent—falsely thought that pregnancy is dated from conception or in the weeks since the person last had sex. Less than one-fourth of the respondents answered both pregnancy knowledge questions correctly.

In our survey, we also asked respondents whether they support a six-week abortion ban. Similar to other national surveys, we find that most Americans oppose strict abortion restrictions—only 35 percent support six-week bans.

Importantly, we find that those who support six-week abortion bans are significantly less likely than others to correctly understand the timing of pregnancy. The statistically significant relationship between having low levels of pregnancy timing knowledge and support for a six-week abortion ban holds in analyses controlling for potentially confounding variables.

Some anti-abortion lawmakers have demonstrated their ignorance about pregnancy before.

There is, for example, a long history of some anti-abortion politicians saying, incorrectly, that it is extremely rare for a person who is raped to get pregnant. Our survey shows that a large swath of those opposing abortion lack knowledge about the basics of pregnancy.

A gender disparity

Perhaps not surprisingly, women in this survey knew more about pregnancy than men. The question about how pregnancies are dated, for example, was answered correctly by 43 percent of women compared with only 23 percent of men. As mentioned above, a majority of Americans incorrectly believe pregnancy is dated from conception, but significantly more men than women think this is true.

This finding is particularly important when considering the gender breakdown of the Florida state lawmakers who approved the six-week ban. Although we do not have data on the pregnancy knowledge of those legislators, we do know that those who voted for the ban were overwhelmingly men.

Florida’s six-week ban will make it much harder for anyone to get abortions there—and it will also affect people in neighboring states who want or need an abortion. In 2023, Florida was home to the closest abortion clinic for 6.4 million women living in the South. In 2023, around 7,700 women from other Southern states, where abortion is now largely banned, traveled to Florida to get abortions.

Overall, our findings raise serious questions about whether Americans without medical training—much like those in our state legislatures—have the necessary knowledge needed to regulate abortion access.

The Conversation