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Why is a 6-week abortion ban nearly a total ban? It's about how we date a pregnancy

The medical community dates pregnancy to the first day of a woman's last period, even though fertilization generally happens two weeks after that. It's a long-standing practice but a confusing one.
Nikola Stojadinovic
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Getty Images
The medical community dates pregnancy to the first day of a woman's last period, even though fertilization generally happens two weeks after that. It's a long-standing practice but a confusing one.

Florida's abortion ban after six-weeks gestation is in effect as of May 1. That means the time a person has to decide whether or not to have an abortion in Florida is – at most – two weeks.

What?

It has to do with how the medical community dates a pregnancy. Here's the deal:

  • A pregnancy is measured from the first day of a woman's last menstrual period, or LMP.
  • For the first two of those six weeks – before ovulation – there is no pregnancy. 
  • Ovulation, sexual intercourse and conception have to happen, generally, about two weeks after the first day of the LMP.
  • There's about a week between when an egg is fertilized and when it implants in the uterus. 
  • It takes another week before there's enough of a specialized pregnancy hormone in someone's urine for a home pregnancy test to turn positive. That's also about the time that a missed period might clue a woman in that she might be pregnant.
  • All of the above is true for people with regular menstrual cycles. For the many women with irregular cycles, it can take longer to diagnose a pregnancy.


So, a six-week limit on abortion is really a limit of four weeks after conception, and one or two weeks after a person learns they are pregnant. There are lots of variations in these biological norms from person-to-person and even from month-to-month.

Florida's six-week ban has exceptions if the pregnant patient's life or a "major bodily function" is in jeopardy, in cases of rape, or if the fetus has a "fatal fetal abnormality."

The six-week limit on abortion is not common – it is in effect in South Carolina and Georgia, at the moment. Many more states have full abortion bans.

The first six-week ban went into effect before Roe v. Wade was overturned in September 2021 in Texas. That was a ban on abortion after a "heartbeat" could be detected. Physicians have pointed out that there is no fetal heart at six-weeks gestation, but cardiac activity begins at that stage of development.

Copyright 2024 NPR

Selena Simmons-Duffin reports on health policy for NPR.