Currently in the UK it is thought that around 33% of twins are Monozygotic (’identical’) and around 66% are Dizygotic (’non identical’ or ‘fraternal’)
Monozygotic Twins (normally referred to as Identical or ‘ Id’ or sometimes ‘MZ’). Mono means ‘from one’ so Monozygotic twins started off as one fertilized egg (zygote) which split in the first few days to a week or so after conception. Monozygotic twins are the same sex* and generally look very alike, hence the label ‘identical’.
Of the Monozygotic (’identical’ ) Twins 33% are Dichorionic Diamniotic (i.e. NOT sharing a placenta or an outer membrane (chorion) This is sometimes abbreviated to DC/DA . This occurs because the fertilized egg split before the chorion had formed, in the first 3 days after conception. Depending how close together the twins implant into the uterus, they may have a fused placenta or or may have individual placentas.
Therefore it is possible for same sex twins to be identical and NOT share a placenta or chorion or amniotic sac66% of Monozygotic Twins are sharing an outer membrane (chorion) but have individual amniotic sacs. This is known as Monochorionic Diamniotic sometimes abbreviated to MC/DA These have split between days 3 and 9 after conception.
1% of Monozygotic Twins are sharing both a chorion and amniotic sacs. These are known as Monochorionic Monoamniotic. This is sometimes abbreviated to MC/MA Monochorionic monoamniotic twins split after day 9 will be monochorionic monoamniotic.