Michelle Emblenton, biochemist at inviTRA, tells us in this video the differences between twins and twins:
Dizygotic twins occur when two eggs are fertilized by two different sperm and the two resulting embryos implant into the mother's uterus. These are often known as fraternal twins or non-identical twins. They're not genetically identical and can be of different sexes depending on which sperm fertilize the egg. They may be alike physically but they won't be identical, it may be alike as siblings. Fraternal twins generally account for about 70% of the cases of twins.
In contrast, monozygotic twins are formed from a single egg and a single sperm which, after fertilization, splits into two separate embryos which then grow together in the mother's uterus. As they are formed from one egg on one sperm, they're genetically identical (hence the term identical twins). So, they will always be of the same sex and usually grow to be virtually identical in every way. These account for approximately 30% of the cases of twins. In the case of identical monozygotic twins we can actually classify them further depending on when the split of the embryo occurred. The time in which the embryo has divided into two individual embryos can affect whether they share a placenta or an amniotic sac. If the embryo divides early, at the morula* stage, it is likely that each embryo will then go on to form its own placenta and its own embryonic sac. The technical term being: bi-chorionic - biamniotic* pregnancy.
If the embryo splits a little bit later, between days four and eight, each embryo will develop its own amniotic sac but will share a placenta. Hence, the technical term: monochorionic biamniotic pregnancy. After this, if the spit occurs a little bit later (perhaps from day 13) it may be that the two embryos will share both the placenta and the embryonic sac. Hence, the term: monochorionic - monoamniotic* pregnancy. If the split occurs after an advanced stage of embryonic development (after day 13) it may be that the split of the embryos is not fully complete and this results in what we know as conjoined twins, where the twins are actually joined in some way physically within their body, as well as sharing a placenta and an amniotic sac. Dizygotic twins being made of separate eggs and sperm, and therefore separate embryos, will always form their own individual placenta and an own amniotic sac. Hence the term: bichorionic - biamniotic pregnancy.
