Your twins could be either...If there is definately only one placenta, then they are identical. But two placentas can become fused and then look like only one placenta, especially on u/s. (Fusing can happen in the early early weeks, so it's possible for that to already be the case at 18 weeks.)
If the twins are b/g (boy/girl), then they are definately dizygotic (two eggs). If they are b/b or g/g then sometimes the only way to know is through zygosity testing. (Zygosity testing is an easy process involving swabbing the inside of their mouths with a q-tip-like-thing, and sending it to a lab---they evaluate the genetic material from there and let you know.)
DZ (dizygotic, two eggs, fraternal) twins can be either b/g, b/b, or g/g, and each have their own placentas, their own chorion, and their own amnion.
MZ (monozygotic, one egg, or identical twins) are always the same sex and can have (a) their own placentas (which can become fused), their own chorions, and their own amnions, (b) one placenta, one chorion, and two amnions, or (c) (and most rare) one placenta, one chorion, and one amnion.
What the tech said is confusing because "in the same sac" means different things to different people. The amniotic sac is typically though of as being just one bag of waters, but it really has several layers. Establishing whether or not they share the same chorion and amnion is the important thing.
mama to a 3.5 yr old boy and twins? (expected in April 2004)