Volunteers with Personhood Colorado had said they collected nearly 4,000 more signatures than the 76,047 needed by the Feb. 12 deadline.
On Wednesday, Secretary of State Bernie Buescher announced that a random 5-percent sample selected by computer found 969 invalid signatures. When using that sample to project the total number of valid signatures, the Secretary of State's Office determined only 60,357 were valid.
The proposal would change the definition of humans in the state constitution to include fertilized embryos.
The Secretary of State's office has given supporters another 15 days to collect sufficient signatures. The new deadline is 3 p.m. on March 18.
A similar ballot issue failed in 2008.
(KUSA-TV © 2010 Multimedia Holdings Corporation)