A monozygotic twin
pregnancy arises from the fertilisation of one egg by one sperm. This
egg then divides into two embryos, resulting in two fetuses.
These twins are identical twins.
This single fertilised egg or zygote may divide into two embryos at
any stage between 1 and 13 days after fertilisation. This process of
division is called cleavage. The type of twin pregnancy depends on when
cleavage occurs:
If cleavage occurs early between 1 and 3 days, the
pregnancy develops in a similar fashion to dizygotic twins
Between 4 and 8 days,
the chorionic sac has already developed and therefore the twins are
only separated by two amnions
Between 8 and 13
days: by this time, the amnion has developed and therefore the
babies are contained in a single sac
Between 13 and 15
days: cleavage may occur at this time, by which point the
development of the embryo has already commenced – and this results in
the very rare conjoined twins
Cleavage at 1- 3 days
If cleavage occurs early (days 1-3 after fertilisation), the pregnancy
develops to have two chorionic sacs and two amniotic sacs. The
development of two chorionic sacs means there are two separate placentas
and each fetus has its own placental blood supply. The placentas can be
in similar or different positions. The development of two amniotic sacs
means there will be a thick intertwin membrane separating the two
fetuses. This is described therefore as a dichorionic-diamniotic (DCDA)
pregnancy.
The diagnosis of whether there are two chorionic sacs and two amniotic
sacs depend on the demonstration of the thick dividing membrane and the
wedge of tissue between the two sacs. This triangle of tissue is well
seen on early scans of twin pregnancies.
Cleavage at 4-8 days
A single zygote results in a monochorionic-diamniotic (MCDA)
pregnancy when cleavage occurs between day 4-8 after
fertilisation.
Here, the pregnancy develops to have one chorionic sac and two
amniotic sacs. The development of one chorionic sac means there is only
one placenta shared between the two fetuses. The development of two
amniotic sacs means there wil be a thin inter-twin membrane
separating the two fetuses. Where the membrane joins the uterine wall in
MCDA twins, the appearance is that of the letter 'T'.
Later in pregnancy, it may be very difficult to see the thin membrane
of a MCDA twin pregnancy.
Here, the pregnancy develops to have one chorionic sac and two
amniotic sacs. The development of one chorionic sac means there is only
one placenta shared between the two fetuses. The development of two
amniotic sacs means there will be a
thininter-twin membrane separating the
two fetuses. Where the membrane joins the uterine wall in MCDA twins, the
appearance is that of the letter 'T'.
Later in pregnancy, it may be very difficult to see the thin membrane
of a MCDA twin pregnancy.
Here, the pregnancy develops to have one chorionic sac and two
amniotic sacs. The development of one chorionic sac means there is only
one placenta shared between the two fetuses. The development of two
amniotic sacs means there will be a thin inter-twin membrane
separating the two fetuses. Where the membrane joins the uterine wall in
MCDA twins, the appearance is that of the
letter 'T'.
Later in pregnancy, it may be very difficult to see the thin membrane
of a MCDA twin pregnancy.
Cleavage at 8-13 days
A single zygote results in a monochorionic-monoamniotic (MCMA)
pregnancy when cleavage occurs between day 8-13 after
fertilisation.
As the division occurs later, the pregnancy develops to have one
chorionic sac and one amniotic sac. The development of a single chorionic
sac means there is only one placenta shared between the two fetuses. The
development of one amniotic sac means there is no inter-twin membrane
separating the two fetuses as both fetuses develop within the single
amniotic sac.
As the division occurs later, the pregnancy develops to have one
chorionic sac and one amniotic sac. The development of a single chorionic
sac means there is only one placenta shared between the two fetuses. The
development of one amniotic sac means there is no inter-twin
membrane separating the two fetuses as both fetuses develop
within the single amniotic sac.
Reproduced with permission from
Dr A Kurtz, Thomas Jefferson University Hospital, Philadelphia
Cleavage at 13-15 days
Conjoined twinning occurs when cleavage of a single zygote occurs
after 13 days. This late division of the zygote develops into a single
chorionic and amniotic sac as in a MCMA pregnancy. The embryos fail to
develop separately and are abnormally fused due to this late division of
the zygote.
This is a very rare form of twin pregnancy and occurs in about 1 in
100 000 births.
We will not consider it further in this session. The
diagnosis, determination of the extent of fusion and the management is
undertaken in highly specialised centres.
Fig 1 Conjoined
twins - a case of thoraco-omphalopagus (reproduced with permission from
Dr Joe Antony)
