A woman in Iowa on Thursday gave Republican presidential candidate Mitt Romney a lesson in why his support for a state constitutional amendment to define life as beginning at conception would have effectively ended up banning many forms of birth control.

Earlier this month, the candidate told Fox News host and evangelical Christian Mike Huckabee that he would have supported a state constitutional amendment to ban abortion if it would have prevented abortions from being covered by the health care law he enacted while serving as the governor of Massachusetts.

"Would you have supported a constitutional amendment that would have established definition of life beginning of life at conception?" Huckabee asked.

"Absolutely," Romney replied.

At a town hall event in Sioux City, Iowa Thursday, a woman told Romney that she was concerned about what this meant for hormonal birth control.

"That would essentially mean banning most forms of birth control," she noted. "Ninety-eight percent of American women, including me, use birth control. So, could you help me understand why you oppose the use of birth control?"

"I don't," Romney declared. "Life begins at conception; birth control prevents conception."

"What I believe is the right course as regards to abortion and life is that I would like to see the Supreme Court return this right to the states and let states create their own legislation with regards to life. That's my view. And states will make different decisions which is their right to do so. And my view is that I'm not out campaigning for an amendment of some kind. I am campaigning to see justices ultimately appointed to the Supreme Court that will follow the Constitution, return to the states the right to make decision themselves."

Romney's plan to "return the right to the states" would allow them to enforce life-begins-at-conception laws, effectively banning the forms of birth control that he claims not to be against.

"I don't know if you want to have some staff look into this, but hormonal forms of birth control work a little differently," the woman pointed out. "They actually prevent implantation, not conception. So, it would ban hormonal forms of birth control which is what most women use."

"As someone who uses birth control, this is a very terrifying prospect for me so I hope that you can, you know, look into that."

In fact, many anti-abortion advocates define conception and fertilization as the same thing.

"At the moment of conception, a male sperm unites with a female ovum," according to the Pro-Life Action League. "After fertilization, the tiny human being travels down the fallopian tube. Implantation, which occurs 8 to 10 days after fertilization, refers to the point at which the baby (now scientifically referred to as an "embryo"), implants in the mother's uterus and begins to draw nourishment."

A 2005 Guttmacher Institute report found that 18 states, including Massachusetts, defined pregnancy as beginning with fertilization or conception.

"[I]t is likely that the proponents of the state laws may have been unaware of how the various contraceptive methods actually work, and were probably not taking aim at them directly," the report states. "On the other hand, many in the antiabortion movement clearly understand the modes of action for contraceptive methods, especially the hormonal methods. Understanding that, they have to know that the end result of enforcing a definition that pregnancy begins at fertilization would implicate not just some hormonal methods, but all of them."

The fringe anti-abortion group Personhood USA has recently been successful at getting more states to take up their legislation that defines life as beginning at fertilization.