How is it that an ovarian dermoid cyst can just show up in my 30s? had ultrasounds during a pregnancy a few years ago and it showed no cysts then.

Stuff happens: Dermoids often grow. In fact they can contain growing teeth and hair. They can also enlarge by bleeding and rarely malignant change. Often dermoids are worrisome for cancer if the patient has an unusual inherited condition, tuberous sclerosis; otherwise cancer in dermoids is really uncommon. Follow up by U/S or laparoscopy is often done.

Cyst can come anytim: Dermoid may have been hidden by the pregnancy. Did the tech look at your ovaries or just the fetus. Get the reports to be sure. Dermoids can grow at any time, may have been missed because it was very small.