Desires lead to deception, and deception leads to disobedience.
(James 1: 15). James uses the analogy of conception, pregnancy, and birth because the birth process so closely parallels the process he is talking about.
When an illegitimate desire is welcomed and acted upon, that act of conception produces a child called sin. And once a child has been conceived, its birth is sure to follow. In other words, committing disobedience is like the act of procreation. The result will always show up after a while.