In Wisconsin, you could try to claim necessity if you were speeding due to a medical emergency:
Pressure of natural physical forces which causes the actor reasonably to believe that his or her act is the only means of preventing imminent public disaster, or imminent death or great bodily harm to the actor or another and which causes him or her so to act, is a defense to a prosecution for any crime based on that act, except that if the prosecution is for first-degree intentional homicide, the degree of the crime is reduced to 2nd-degree intentional homicide.
However, this might or might not work. Speeding is not technically a crime; it's a civil forfeiture. The 1982 Wisconsin Supreme Court case State v. Brown held:
We need not and we do not decide whether a defense of legal justification is available to the defendant in a civil forfeiture action for speeding if the causative force is someone or something other than a law enforcement officer. We decide only that a defendant in a civil forfeiture action for speeding may claim that his violation of the law should be excused if it was caused by the state itself through the actions of a law enforcement officer.
So, if you made such an argument, the courts would have to decide whether to accept it. In the above case, two justices out of the six who heard the case said in a concurring opinion that "the public interest in highway safety and the expeditious enforcement of traffic laws outweighs an individual defendant's interests of a personal nature in any other circumstances." (I strongly disagree with characterizing the avoidance of imminent great bodily harm or death as a mere "interest of a personal nature", but they said what they said.)
Also, in the case of a woman in normal labor, it's not clear that there will be "imminent death or great bodily harm" if you don't speed. Your belief that speeding is the only means to prevent this must be "reasonable" given the circumstances, or necessity would not apply even if the courts decided that this defense could apply to speeding.