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 developed.
• 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 years.
• 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 Detail
• 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
people.
• Some MZ twins have two placentas while
some share one.
• 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.
| |
| Baby
on the way! |
|
|
| |
The Sub-types
Mirror-image Twins
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.
Conjoined Twins
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.
