Summarize this article:

Preterm labor is more common when you're delivering twins. If you notice any of the following symptoms, contact your doctor immediately.  Bleeding or vaginal discharge can be a sign of preterm labor. You may also start having contractions early. Diarrhea can sometimes signal preterm labor. Preeclampsia is more common during a twin pregnancy. This is a spike in blood pressure that occurs during pregnancy. Make sure to have your blood pressure monitored throughout pregnancy to make sure it's at a healthy and safe level. Rapid weight gain or headaches should be evaluated by your doctor immediately. These can be a sign of potential complications, and indicate preeclampsia is developing. Your doctor may need to deliver the twins immediately to assure their safety.

Summary:
Watch out for signs of preterm labor. Monitor your blood pressure closely. Have certain symptoms evaluated immediately.