How long until I tell work about being pregnant?

Nakita • 30 years old, Married Baby 1- June 2015, Baby 2- October 2016, Baby 3- March 2018! Baby 4 October 2019
I work in a very physical environment and I know I am very early on in my pregnancy. I work at Purolator and I pack freight that goes into planes sometimes lifting 75lb + boxes. When should I let work know I'm pregnant