"We thus strike a balance between the interests of the pregnant woman and the interests of the state in health and in potential life. Fortunately, these decisions come at a time when a majority of the legislatures of the states are in session. Presumably where these decisions cast doubt as to the constitutional validity of a state’s abortion statute, the legislature of that state may immediately review its statute and amend it to bring it into line with the constitutional requirements we have endeavored to spell out today. If this is done, there is no need whatsoever for any prolonged period of unregulated abortion practice."