Most doctors count how many weeks a pregnancy has progressed starting from the woman’s last period (Image: Cristian Baitg/Getty Images)

On 12 April, the governor of Arizona approved a bill banning most abortions after 20 weeks of pregnancy. However, the bill marks pregnancy as beginning two weeks before a child has been conceived. On the face of it, this sounds like nonsense, but what does the science say? New Scientist takes a closer look.

What does the bill actually say?

The bill bans the abortion of a fetus that is at or over 20 weeks of gestation, except in cases of medical emergency. It also states that gestational age should be defined as “the age of the unborn child as calculated from the first day of the last menstrual period of the pregnant woman.”


That starts the fetal clock an average of two weeks before the fetus actually exists. The purpose of a menstrual period is to get rid of an unfertilised egg, plus all the tissue that has built up in the womb to support it. A new egg typically reaches the uterus two weeks later. In practice, the law therefore bans abortions as early as 18 weeks into the fetus’s development.

Does that definition of gestational age make any sense?

More than you might think. Most doctors count how many weeks a pregnancy has progressed starting from the woman’s last period.

“It’s been the convention for generations to measure the length of pregnancy from the first day of the last period,” says medical ethicist Farr Curlin of the University of Chicago, Illinois. He says it is hard for women to pin down what day fertilisation may have occurred, but can easily remember the first day of their last period.

When does a pregnancy actually begin?

“It would be absurd to say a woman is pregnant who is having her period,” Curlin says. However, there is surprisingly little consensus in the medical community about whether a pregnancy begins at the moment the egg is fertilised by the sperm, or when it is implanted in the uterus.

The American College of Obstetricians and Gynecologists opts for implantation, in light of the high percentage of fertilised eggs that never successfully implant.

But according to a recent survey conducted by Curlin and colleagues, 57 per cent of obstetricians and gynecologists believe pregnancy should be defined as starting at fertilisation. They also found that, even for practising doctors, personal beliefs about religion and ethics swayed their opinions: doctors who don’t oppose abortion are more likely to say pregnancy begins at implantation; doctors against abortion are more likely to prefer conception.

Do similar abortion laws exist elsewhere?

Since 2010, six other states have passed laws banning abortion after 20 weeks of pregnancy unless the mother’s life is in danger. However, they all date pregnancy starting from fertilisation. Other states vary around the 24 to 26-week mark.

In the UK, the law states that most abortions can be carried out up until 24 weeks.

“When the Arizona bill was introduced using a different, earlier point to start pregnancy, it was pretty clear to people who have read these bills that the goal was to ban abortion two weeks earlier and still be able to call it a 20 week ban,” says Elizabeth Nash of the Guttmacher Institute, a policy analysis group advocating sexual education.

What evidence is Arizona’s new law based on?

The idea behind banning late-term abortions is to avoid terminating a fetus that could survive outside the womb – a so-called viable fetus. A 2001 study found that 75 per cent of fetuses could survive at 25 weeks, but none survived at 21 weeks, counting from the last menstrual period.

“No baby born at 20 weeks gestation has ever survived anywhere in the world,” says John Lantos of the Children’s Mercy Hospital in Kansas City, Missouri.

The Arizona law also states that, based on “strong medical evidence”, an unborn foetus can feel pain during an abortion at 20 weeks. The issue of when a fetus can feel pain has been hotly contested. However, it is widely considered that brain pathways responsible for the perception of pain are not thought to be complete until 26 to 29 weeks.