From the standpoint of your question as posed, there's no "continuum" involved. Once "it" is a living human being -- i.e., at conception, which is not arbitrary at all -- then "it" is a person and should have rights at that time.
Strictly speaking, all biological phenomena are on a continuum. Our need for discrete terminology doesn't trump the laws of physics. So, of course you are wrong. There are a continua of chemical processes involved in the process of conception (albeit over a short period compared to gestation time).
Still, what criteria determine rights?