Medical professionals are fighting a ballot initiative that would define when life begins.Opponents of a controversial constitutional amendment gathered on the west steps of the state capitol Tuesday, urging Coloradans to avoid signing a petition that would put the "personhood" proposal on the November ballot.The measure would define "personhood" as the beginning of conception. Opponents worry the permanent addition to the state constitution would open a Pandora's Box."The moment of fertilization is not a medical definition and is almost impossible to determine outside of a petri dish. So defining a person that way interferes with a lot of things including the practice of medicine," said Dr. Mary Fairbanks, a family practitioner. "Medical providers who treat women of reproductive age would be at heightened risk of lawsuits if the care they provide could potentially affect a woman's fertilized egg."Another doctor, OB/GYN Dr. Andrew Ross, said birth control access could also be threatened. Ross said birth control alters the lining of the uterus where the egg implants and this measure could say that threatens a fetus' right to life.Critics say the personhood amendment opens the door to making abortion illegal.Kristi Burton, 20, launched the campaign and believes every human life has equal value under Colorado's laws. She believes those rights should extend themselves to a fertilized egg. As of three weeks ago, 68,000 people agreed.Burton will need 76,000 registered voters to sign a petition to get the issue on the ballot. Burton has until May 13 to get that done.