Ok, I've had a lot of questions about the different kinds of twins, so I thought I would explain a little. I've learned alot about twins lately, so here you go.
Fraternal twins are the result when two different eggs are fertilized by two different sperm. This leads to the development of two separate placentas, each with its own chorion and amnion.
Fraternal twins are more common than identical twins and account for about 2/3 of twin pregnancies.
Fraternal twins are more common when:
-there is a family history of twins
-the mother is 'older'
-the mother has had many previous pregnancies
-fertility treatments were used
-sometimes they just 'happen'
Identical twins develop when a fertilized egg splits. Depending on when the split occurs will determine if the twins share a placenta, with either one or two chorions and amnions, or if they each develop their own placentas. In general, the later the split occurs, the more likely that the twins will share one placenta.
Identical twins are genetically identical, therefore, are always the same gender.
Identical twins are more commone when:
-Nothing! Identical twins are spontaneous and there is no research showing identical twins are linked to anything specific.
Our twins do have separate sacs, as you can clearly see in the ultrasounds pictures, but my doctor cannot definitively say whether they are fraternal or identical. Identical twins can either share a sac or have their own, depending on when they split. If we find out they are boy/girl twins, then we will definitely know they are fraternal. If they are boy/boy or girl/girl, we will never know unless we get a blood test after they are born to see if they are identical.