Fighters who have missed weight are now 6-0 in 2018.
Dern made her UFC debut at UFC 222 defeating Ashley Yoder by split decision.
Cooper made weight and the fight will go on as planned at a 123-pound catchweight, with Cooper receiving 30 percent of Dern's purse as a penalty. "She's not getting out of this fight that easy".
Make what you will of her weigh-in on Friday, but on Saturday night Mackenzie Dern put her skills on display. Cooper started the fight aggressively, taking control of the center of the octagon and landed left hands.More news: Hawaii's Kilauea volcano fissures observed from NASA satellites
"When I got here, I was okay, but the people in the UFC, they were a little bit anxious", Dern told MMA Junkie earlier this week.
Dern looking to find a right home for her right hand.
As the fight got underway, Dern showed confidence in her hands by exchanging with Cooper nearly immediately and not backing down from a more experienced striker on the feet.
UFC women's strawweight standout Felice Herrig also took to social media to slam Dern.