More than half of US states will outlaw abortion immediately or as soon as practicable, if a leaked draft decision from five supreme court justices remains substantially unchanged.

The result would send hundreds of thousands of people in 26 states hostile to abortion elsewhere to terminate a pregnancy – either by traveling hundreds of miles to an abortion clinic or seeking to self-manage abortion through medication from grassroots or illicit groups.

Many would also be forced to carry unwanted pregnancies to term.

“Abortion is an essential part of reproductive healthcare, and this is going to affect people, even people who think, ‘I will never have an abortion,’” said Dr Nisha Verma, a Darney-Landy fellow with the American College of Obstetricians and Gynecologists.

On Monday, a draft supreme court decision in arguably the most contentious case in generations was leaked. The case considered whether Mississippi could ban abortion at 15 weeks gestation.

The ban is highly significant because it strikes at the heart of US constitutional protections for abortion. The landmark 1973 decision Roe v Wade established the right for pregnant people to terminate a pregnancy up to the point a fetus can survive outside the womb, roughly considered 24 weeks gestation, and a legal principle called “viability”.

The decision invalidated dozens of state bans, and until the court issues a final decision, prevents states from outlawing abortion before viability. A final decision is expected from the court in late June.

The leaked decision in the Mississippi case, called Dobbs v Jackson Women’s Health Organization, shows five conservative justices are willing to reverse constitutional protections for abortion on the grounds Roe v Wade was wrongly decided.

If the decision is not substantially changed by the time a final opinion is issued, abortion regulation would be returned to the states where lawmakers across the south and midwest of the US have enacted bans in anticipation of the court’s decision.

“There’s six months to two years before the dust settles,” said Elizabeth Nash, interim associate director of state issues in the Guttmacher Institute, a reproductive rights research organization. “It will be chaos.”

In that time, “there will be a lot of fluctuation as states are trying to implement their bans”, some of which are designed to go into effect immediately after a court decision is issued.

Such state bans would probably close abortion clinics for nearly half of US women of reproductive age (41%) and increase the average driving distance to an abortion provider from 35 miles to 279, according to predictions from Professor Caitlin Knowles Mayers, an economist at Middlebury College in Vermont who has studied how the reversal of Roe would affect accessibility of abortion. This would probably reduce the rate of abortions by 20% in states that ban the procedure and increase births by 4% (birth estimates are less certain).

“As was the case in the pre-Roe era, many women seeking abortions will find a way to get to the states where abortion is legal,” said Myers. “Current empirical evidence suggests that about three-quarters of women in the states that go dark will manage to make such a trip, reaching providers in soon-to-be “border” states like Florida, Illinois, New Mexico and Virginia.” Myer also works as a consultant to the Center for Reproductive Rights and Planned Parenthood Federation of America, one of the nation’s largest networks of abortion providers.

Roughly 860,000 induced abortions are performed each year in the US. However, a disproportionate share of the people who seek abortions are low-income or people of color who already have children, making it more difficult to obtain an abortion.

“Current evidence on the causal effects of travel distances indicates that about one-quarter of women seeking abortions will not be able to travel to obtain them and that most of these women end up giving birth as a result,” said Myers.

What’s more, as women from states where abortion is illegal travel to states that allow the procedure, it could push up wait times for abortions in states where it is legal and perhaps send people seeking to terminate pregnancies even farther – like a secondary wave of medical refugees.

For example, California could see a nearly 3,000% increase in the number of people whose nearest legal abortion clinic is in the state. That could increase patient load from roughly 46,000 people a year to 1.4 million a year, practically overnight.

Already, most people who seek to terminate a pregnancy do so through medication abortions. Medication abortion is a two-step protocol that safely terminates pregnancies up to 10 weeks gestation.

Activists have already set up networks to mail abortion pills to women in states where abortion is illegal to self-manage an abortion. However, obtaining that medication would be illicit in states where the procedure is outlawed.

In addition to the immediate effects on abortion, a decision giving control of the issue back to states would also substantially alter obstetrics care.

“We find these laws are so extreme, clinicians are scared to provide evidence-based care,” said Verma. “They don’t know if they’re going to be prosecuted or fined or criminalized.”

Nearly half (44%) of all future obstetricians and gynecologists are trained in states where abortion could become illegal, making it difficult to train young new doctors in care for early miscarriages, which use the same skills as induced abortion, or for ectopic pregnancies. The US already has among the highest rates of maternal mortality in high-income developed nations.

“This goes beyond induced abortion,” said Verma. “It’s going to affect people across the spectrum.”