Unless you have had twins yourself or know twins, you may not know that there are 3 different types of twins.
Dizigotic or fraternal Twins (non-identical)
These types of twins consist of 2 eggs that have been fertilized by 2 different sperm that have implanted at the same time. This is the most commonly occurring type of twin pregnancy.
All dizygotic twins are dichorionic, which means they have two separate sacs and two placentas.
Monozygotic (identical twins)
Monozygotic twins or identical twins are the result of when one egg and one sperm splits to create 2 individual embryos. Monozygotic twins usually have the same chromosomes and can also be called maternal twins.
Monozygotic twins can be either dichorionic/diamniotic (2 separate outer sacs and two inner sacs), monochorionic/diamniotic (one outer sac and 2 inner sacs) or the rarest form of twins monochorionic/monoamniotic (both embryos share both sacs).
Polar body twins
Polar body twins happen very rarely, and they are the result of one egg fertilized by two different sperm. Polar body twinning would result in "half-identical" twins.
When you attend an early pregnancy scan or if you don't have an early scan when you attend your dating scan some people learn that they are having two or sometimes three babies.
A question that many people will ask if they are told that they have a multiple pregnancy is, "Are they identical?" This can be answered on only some cases using ultrasound.
If the scan shows that the embryo's are monochorionic/diamniotic (one outer sac and 2 inner sacs) or monochorionic/monoamniotic (both embryos share both sacs) then we can confirm that you have identical twins. If the sacs are dichorionic/diamniotic (2 separate outer sacs and two inner sacs) then you could still have identical twins but the split in to two individual embyo's could have happened very early after fertilization.
We can never be fully sure if the twins in 2 separate sacs are identical unless they are differing genders. You can find out at a gender scan but not until about 16 weeks or earlier if you have other testing. There are rare variations of this but it could get extremely complicated so we will stick to the basics.
Take a look at the illustration below to show the differences in twin pregnancies.