"Pain during labor is caused by contractions of the muscles of the uterus and by pressure on the cervix. This pain can be felt as strong cramping in the abdomen, groin, and back, as well as an achy feeling."

https://kidshealth.org

The muscles are fully capable of performing the action. The contractions and spasms are the pain, as your body prepares them for what they need to do.

Besides, it's not actually the uterus that needs to contract, it's the cervix. And we have no way of knowing if elastagirl has manual control over her cervix. Most women don't have control over their cervix, hence why these contractions have to happen for childbirth to take place. It could also be risky to the fetus to try to stretch enough to make that happen.