Most adnexal masses in pregnancy are diagnosed incidentally in the first trimester. Most of these are benign, asymptomatic and spontaneously disappear by 16 weeks of gestation. But some of the masses may persist and may cause serious problems, such as torsion, rupture, obstruction to vaginal delivery and delay of malignancy diagnosis. Ultrasonography is the first-line imaging modality used for evaluating adnexal masses. Ultrasound characterization of the complexity and size of the adnexal mass can assist with the prediction of malignancy. Making the decision to operate an adnexal mass diagnosed in pregnancy is often difficult. Some investigators recommend expectant management and others recommend surgical management. The obstetrician must balance the risks and benefits of surgery or observation by taking both mother and fetus into account. In this report, we present a case of huge, multiloculated ovarian benign cyst in twin pregnancy which grew rapidly during the second trimester, like a malignant mass and managed surgically. Copyright © 2014 by Türkiye Klinikleri.