2 children born at the same time.
Unfortunately, the answer to this question is still very much unknown. However, non-identical twins are more likely to occur if
There are 2 types of twins:
Monozygotic twins occur when one egg is fertilised by one sperm and then at some time during the first 14 days, this fertilised egg splits. Therefore, all identical twins have the same DNA and therefore are always the same-sex. Around one-third of twins in the UK are monozygotic.
There are different types of monozygotic twins, depending on how early or late the egg split. in basic terms, if the egg splits within the first 3 days of fertilisation (whilst it has not yet embedded in the lining of the womb), there will be 2 placentas and 2 sacs. If however, the egg splits later there may be 1 placenta and 2 sacs or 1 placenta and 1 sac. Also occasionally, 2 placentas may fuse together, so it appears as if there is only 1 placenta.
It is a bit complicated, but a consultant described it to me like this:
the sac is the house and the placenta is the food. So with 2 placentas and 2 sacs, the babies have their own house each and their own food each. With 1 placenta and 2 sacs, the babies share their food but have their own house. With 1 placenta and 1 sac, the babies share their food and share their house.
Just a note to say from experience, you cannot tell if you have identical or non-identical twins of the same-sex with 2 placentas and 2 sacs. During my pregnancy I was told I was having non-identical twins as they had 2 placentas and 2 sacs. However, they have always looked very similar, so after 18 months we had a DNA test done and it confirmed that actually they are identical twins! The only way to know for sure is to have a DNA test.
Dizygotic twins or also called fraternal twins occur when 2 individual eggs (released at the same time) are fertilised by 2 separate sperm. These twins have the same percentage DNA of each other as any other sibling born at different times.
Around two-thirds of twins are non-identical.
Non-identical twins always have 2 sacs and 2 placentas. They may be of different sexes or they may be the same-sex.
Mirror twins are identical twins, that develop when the splits fairly late (9-12 days) and they appear to look the same, but as if looking in a mirror. They tend to have opposite dental problems, cowlicks on opposite sides, one left and one right-handed etc… For more information on twin handedness view my post on left and right-handed and identical twins?