A. Twinning occurs in approximately 1 in 80 pregnancies in North America.
Most of the time we talk about the two most common types of twinning:
Dizygotic twins, or fraternal twins, make up the majority of the twins we see, at about 70% of twins. In North America this type of twinning occurs in about 12 of every 1000 pregnancies. Unlike identical twins, this type of twinning seems to run in the family, and has certain racial tendencies. (Twinning tendencies are passed on through the female descendents of mothers who have had twins).
More to your point, fraternal twins develop from 2 EGGS and 2 SPERM, and most of the time are each found in their own sac within the uterus. Normally a woman produces and releases 1 egg each month in the hopes of becoming fertilized. It is not uncommon, however, for her to release two in the same month, and it is when this happens that fraternal twins become a possibility. Fraternal twins can be as similar or dissimilar as any other siblings.
Monozygotic twins, or identical twins, make up about 30% of the twins
we see. This occurs in about 3 of every 1000 pregnancies and appears to be
a random event (ie does not run in the family).
These twins actually derive from 1 EGG (ovum) and 1
SPERM. These kids generally have the same physical characteristics (eye
colour, build) as well as the same genetic features (blood type). So yes,
they are always the same sex, but remember, they're not TOTALLY the same...
they have different fingerprints!!
TIP: If you plan on making multiple queries, you might wish to make a bookmark to this page.
