With the baby due in roughly 2 weeks (or less) ... a very pregnant Serena used a Sharpie pen to draw on her stomach for a sports-style breakdown of how the birth is gonna go.

It's almost like if John Madden had to draw up and break down the delivery.

After that, Serena honed in on her baby daddy, Alexis Ohanian -- and pointed out the gray hairs on his beard.

Don't worry ... she's got a solution for how to make the gray go away -- and it's really funny!