since when is it legal to abort at 28 weeks?
Since Roe vs. Wade.
When you allow for "health of the mother" exceptions, killing the unborn is probably legal right up to birth in most of our bloodthirsty country.
Since 1973, when Roe v Wade became the law of the land. It became legal to abort up to the moment of birth for any reason in every state in the country. It's only been since 1989 when the Webster decision came out of the Supreme Court that states could begin putting very very limited restrictions back on the procedure. Since 1989, very few states have been able to get laws past their courts.
If in the 9th month of pregnancy, a woman/girl goes into labor, they can go to an abortion clinic to have an abortion, according to the law. States have passed restrictions, but not all states...
Since Roe v. Wade ~ it's been legal to abort a baby at any stage of development ~ you must be confused if you think there's some other point of law involved in this.