My son is 18 months. But yet to speak. All he can is " murm murm" asking for food and "mah mah" . Need to see speech therapist ?

My girl only said Mama at 2.5yo and started talking 3 words sentences at 3.5yo. It's speech delay. But at 4.5yo, doc did not refer to therapist as he said she started talking late. As long as improve.