I, too, have read others' stories about cyst surgery during pregnancy. An 8 cm dermoid might be a bit of a problem during delivery which is why removal is being recommended now, but I wonder if it is possible to just remove it following a scheduled c-section rather than subject you to this now (I'm not a doctor, I am just wondering about alternatives). It's supposed to be safe to have surgery during the second trimester. It is is really important to ask these tough questions of your doctor prior to surgery. Laparotomy can lead to a "down time" of many weeks at home while you recover and your mobility won't be great in those earlier weeks. I assume they are only going after the cyst itself but one must wonder if the ovary is still in good shape. The womb and baby should not be affected, but mommy will be plenty sore herself. Again, ASK THE DOCTOR. Surgery is never to be taken lightly and the patient deserves full knowledge of what to expect.
There have been other women on here who have had a similar situation. I know that questions about this have been asked on the related export forum for ovarian cancer (see below on the right). I would suggest you do a search there on cyst surgery when pregnant. Do you know what kind of cyst and size it is?