Talking to parents about your concerns can be difficult. However, open communication is always best. You may suspect a delay, but after talking to parents you might find there are other underlying reason for what you have observed or you might find parents have the same concerns. This handout from the CDC can be a helpful starting point for a conversation to determine if there is a need for further assessment. Though it was designed more for classroom teachers it has some good advice.
These tips sheets come from the Center for Disease Control Website.