Last week, the Missouri legislature overrode the veto of Democratic Gov. Jay Nixon and passed a statute requiring women to wait 72 hours before obtaining an abortion, even in cases of rape or incest. To be sure, this is not the most important abortion restriction to have recently passed a conservative state legislature — but it is a particularly good illustration of how needless abortion regulations treat women as second-class citizens who are incapable of making decisions for themselves.

For good reason, a great deal of attention has focused on the draconian abortion regulations passed in Texas, which have already caused a wave of clinic closures that have rendered abortion practically inaccessible for women in many parts of the state. As District Court Judge Lee Yeakel found in a brilliant recent opinion, the burdens the state imposes on doctors and clinics do not have a serious public health justification, but are simply backdoor means of cutting off access to abortion.

Indeed, the fact that the restrictions target abortion clinics, rather than applying more broadly to other kinds of clinics, makes it clear that Rick Perry and Co. are not really interested in women's safety. And there is little justification for singling out abortion, since that procedure is actually safer than carrying a pregnancy to term.

The Texas regulations, however, at least have the decency to pretend they're about safety. Waiting periods, on the other hand, do nothing to make the abortion procedure safer, even in theory. As Nixon observed in his veto message, the waiting period "serves no demonstrable purpose other than to create emotional and financial hardships for women."

This gets to a bigger constitutional issue. Mandatory waiting periods — whether it's the more typical 24-hour requirement, or the 72-hour period required by South Dakota and now Missouri — lay bare the incompatibility of most abortion regulations with gender equality.

As Justice John Paul Stevens pointed out in the 1992 case Planned Parenthood v. Casey, mandatory waiting periods rest "on outmoded or unacceptable assumptions about the decision-making capacity of women." The message being sent by state legislatures, essentially, is "you must be crazy if you want to obtain an abortion — maybe think about it some more and you'll come to your senses."

Suffice it to say that safe medical procedures sought by men, such as vasectomies, are not burdened with such waiting periods.

Unfortunately, Justice Stevens' unanswerable argument was made in dissent. While Casey is most famous for refusing to overrule Roe v. Wade, it replaced Roe's clear "trimester framework" with the principle that abortion regulations were constitutional unless they imposed an "undue burden" on a woman's right to choose to have an abortion.

In theory, the "undue burden" standard could still provide robust protection for a woman's right to choose, carefully distinguishing between genuine safety regulations and those that are thinly veiled attempts to make it more difficult for women to obtain abortions.

But when it upheld the mandatory waiting period in Casey, the Supreme Court essentially guaranteed that the "undue burden" test would be ineffective going forward. After all, waiting periods are based on nothing more than the assumption that women aren't thinking clearly, at least according to the predominantly male legislators who pass such laws. If that doesn't constitute an undue burden, what would?

The new three-day waiting period in Missouri does have one element in common with the Texas regulations: They both disproportionately affect the most vulnerable women. Under the Texas statute, while abortion clinics will remain open in the largest urban centers, women in rural areas of the state will have to drive huge distances to find a clinic. Similarly, waiting periods are a manageable annoyance for women in urban areas with flexible work schedules; but for women who have to travel long distances to obtain an abortion, the burden imposed is substantial, indeed.

The new Missouri law, therefore, is a quintessential contemporary abortion regulation: It has no legitimate purpose while imposing the greatest burdens on women who can least afford an unwanted pregnancy. It is inconsistent both with a woman's reproductive freedom and her right to the equal protection of the laws.

The constitutional status of the 72-hour waiting period has yet to be determined by the courts, but it's time to say enough is enough. The new Missouri law, like the Texas law currently under review, constitutes an "undue burden" in every possible sense.