This is some info regarding TWINS. I got from a website..
There are 4 different types of identical twins, depending on the length of time after conception the fertilised egg splits. Each category generally relates to the number of placentas that develop during the pregnancy and the number of fluid sacs that surround the babies. The medical terms used to describe the different identical twin types combines the following syllables:
mono = 1
di = 2
chorionic = the placenta, as well as the fine membrane that lines the uterus, encapsulating the amniotic sac and baby(s).
amniotic = the fluid sac(s) the babies float in.
To describe the categories as follows:
Dichorionic and diamniotic. These types of identical twins develop 2 separate placentas and 2 separate amniotic sacs (similar to fraternal or non-identical twins). They generally occur if the fertilised egg splits before 3 days after conception.
Monochorionic and diamniotic. These identical twins share 1 placenta (monochorionic) but develop 2 separate fluid sacs (diamniotic). This is the most common type, accounting for around 75% of identical twin pregnancies and happens if the split occurs between days 3 and 8 after conception.
Monochorionic and monoamniotic. These twins share 1 placenta (monochorionic) and 1 sac (monoamniotic). This is the rarest type of identical twins, occurring if the split does not occur until 8 to 13 days after conception. Monochorionic/monoamniotic twins are associated with more pregnancy complications for the babies. These are discussed in health variations for multiples.
Conjoined or Siamese twins. In very rare circumstances, the twins do not split until the 13th to 15th days after conception. This leads to the babies themselves being physically joined together to some degree, as well as sharing 1 placenta and 1 amniotic sac (monochorionic and monoamniotic). The joining may just involve the babies skin on some part of their body (being easy to surgically separate soon after birth), or it may involve the babies sharing vital organs such as their brain or heart (making surgical separation more involved or not possible). These babies need to be born by Caesarean, with any proposed surgical separation being performed at some stage after they are born.
Acardiac twins. This is a very rare type of twinning that is similar to conjoined twins, except the babies share one heart. Separation may be possible, but the second twin cannot survive. However, the condition itself is also life-threatening for the first twin because of an overload on the heart trying to support both babies. This can lead to heart failure (in 50% of cases). Again, the babies need to be born by caesarean and any proposed surgical separation is performed at some stage after they are born.
The splitting of a developing baby to create twins is not possible once the pregnancy is more than 15 days after conception.
NOTE: Identical triplets are referred to as 'trichorionic' and 'triamniotic', meaning 3 placentas and 3 amniotic sacs. With different triplet pregnancies having various combinations of sharing placentas or sacs. If 2 babies are identical in a triplet pregnancy, they may share 1 placenta and/or sac, with the 3rd baby having their own placenta and sac.