If you've seen the maps about the status of abortion rights post-Roe, you've likely seen figures suggesting about half the states in America would ban abortion in Dobbs' immediate aftermath. Some of these are just states which are poised to act when Roe falls. Others have so-called "trigger" laws, which would criminalize abortion starting from the moment Roe is overturned.
But in at least a few states, there were laws which pre-dated Roe banning abortion that have never been repealed. And that, to my somewhat untrained eye, presents a big problem for women in those states who may have had an abortion during the Roe era.
When a law is "struck down" as unconstitutional, it is not, as is popularly held, stricken from the books. The law still exists, it is just practically unenforceable. One effect of Dobbs would be to resurrect these zombie laws. But the question is whether the prohibitions found in those laws could be used to prosecute women who had an abortion while Roe was still in effect.
The instinctive answer is no, because the constitution prohibits ex post facto criminal lawmaking. You cannot criminalize conduct retroactively. So a state could not newly criminalize abortion and make that law apply to conduct that occurred before the law was passed. That would characterize many of the "immediate" abortion ban states; including, I think, the "trigger" law states.
But in the case of our states that simply kept their pre-Roe abortion prohibitions on the books, things may be different. The argument there would be that abortion was always illegal in those states, including during the Roe period. Yes, those laws couldn't be enforced during Roe's pendency, but the criminal prohibition was still on the books at the time the woman had the abortion in question. It will not be Dobbs that criminalizes abortion in these states, Dobbs will just remove the barrier that had prevented the state from enforcing its always-operative anti-abortion statute. It's as if you committed a crime but the DA couldn't prosecute because his hands were literally tied behind his back. Once he is freed from restraints, you cannot then say "well, I acted relying on the knowledge that the DA was incapacitated".
Does the rule against ex post facto criminal laws prevent prosecutions in such a case? It is far from clear to me that the answer is yes. Women in states that had continuous abortion bans in place during the Roe era may be at real risk of prosecution (assuming they're within the relevant statute of limitations). Yet another way that overturning Roe will wreak havoc on the settled expectations of millions of American women.