Twins are the development of two embryos in the womb.
There are a few types of twinning but the two most
common ones are fraternal and identical. Fraternal
twinning which by far occurs more commonly is technically
known as dizygotic. Monozygotic or identical twins
are a rarer occurrence.
Dizygotic or Monozygotic?
study of the placenta or membrane alone is not enough
to tell if your twin is dizygotic or monozygotic.
Perhaps you can have an indication but only DNA testing
can provide you with a conclusive answer.
Dizygotic Twinning in Detail
• Dizygotic (DZ) twins involve the union of
two eggs with two sperms, resulting in two embryos.
Two separate babies forms, who are genetically dissimilar
as any two siblings born separately to the same parents.
• Conception of DZ twins occur when the mother
produces two separate eggs in the same monthly cycle
or when more than one embryo is transferred to the
uterus during IVF.
• DZ twins usually have separate placentas which
may at times fuse together resembling a shared placenta
• DZ twins can result because of superfecundation
which is when conception takes place on two different
occasions during the fertility phase. This would mean
one fetus will be a few days older and therefore more
• Fraternal twins run in families through the
mother's side. Some women genetically ovulate more
than one egg per cycle.
• Women who have had more than 3 children are
at an increased tendency to conceive fraternal twins.
• Fraternal twins are more common in mothers
aged 34-39 as ovulation is more erratic during these
• Fifty percent of all fraternal twins are boy/girl
pairs, with twenty five percent being boy/boy and
the other quarter being girl/girl.
Monozygotic Twinning in
• Monozygotic (MZ) twins start off in the same
way as any singleton pregnancy i.e. one egg unites
with one sperm to form one embryo. BUT in the very
early stages of cell division the developing cell
group divides into two and progress as two separate
embryos. It is not established why the split occurs.
• Genetically, MZ twins are coded with identical
DNA. They look very similar but are two different
• Some MZ twins have two placentas while some
• MZ twins have same genes but different fingerprints.
• The chance of having a pair of girls is the
same as having a pair of boys.
• Identical twins do not run in the family,
and so it can happen to anyone.
Mirror image twins have physical features are on the
opposite side of their bodies. One twin being left
handed and the other being right handed is a clear
example. Other obvious examples include having birth
marks or moles on the opposite sides. About 25% of
MZ twins are mirror-image.
Popularly known as Siamese twins, conjoined twins
occur in 1 in 100,000 births or 1 in 200 MZ twins.
Conjoined twins can be identified via ultrasound in
the early part of pregnancy. Apart from sharing placentas
and membranes, conjoined twins also can also share
body parts and organs. Occurrence of conjoined twins
happens when the zygote divides late and doesn't split
totally. The twins can be linked at any part of the
body but both will be joined at the same place.