The Broward State Attorney's Office filed the charge Friday. The 6-foot-5, 305-pound Merling was arrested May 27 in Weston. When the Broward Sheriff's Office arrested him, they noted that he either knew or should have known the woman was pregnant.
If convicted, the 25-year-old could face up to 15 years in prison and a fine of up to $10,000.
The NFL also could punish Merling for violating the league's personal-conduct policy.
A telephone message left with Merling's attorney, who has filed a plea of not guilty, was not immediately returned.
Merling had 33 tackles and 2 1/2 sacks for the Dolphins last season.