In some cases, the cell that is the
result of fertilization may proceed to an external division and
split up into two separate cells. These two new cells then
continue their development independently as they undergo their
own segmentation. Eventually, they both arrive in the uterus
where they also implant separately. The result is a twin
pregnancy. Since both came from the same egg and the same sperm,
the twins will be identical, i.e., they will be of the same sex
and, of course, also look alike in every other respect.
Actually, identical twinning does not necessarily have to begin
in the Fallopian tube, but can occur at any time between the
second and fourteenth day after fertilization. Thus, even during
implantation the blastocyst, which is an early stage of the
human development early in pregnancy, may still divide and
grow into two separate embryos.
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Fraternal twins
Fraternal (dizygotic) twins develop
when separate eggs are fertilized by different sperm. Fraternal
fetuses have separate placentas and amniotic sacs.
Fraternal twin births have
greatly increased since 1980, due to the use of infertility
treatment. Multiple fetuses conceived with assisted reproductive
technology are always fraternal.
Fraternal twins:
- Tend to run in families.
- Are born most frequently to
women who are white or of African descent.
- Can be of different sexes
and have different blood types.
- May look very different from
one another, with different-colored hair and eyes; they may
also look alike, as siblings tend to do.
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Identical twins
Identical (monozygotic) twins and triplets come from a single
egg that has been fertilized by one sperm. For unknown reasons,
the fertilized egg splits into 2 or more embryos during the
first stage of development.
Identical twins may share the
same tissue (placenta) that connects a mother and fetus.
However, they usually are contained within separate amniotic
sacs in the uterus. Very rarely, identical twins may share one
amniotic sac (monoamniotic twins).
Identical twins or triplets:
- Are always the same sex and
blood type.
- Do not always look alike.
- May look like mirror images
of each other. One child may be right-handed while the other
is left-handed.
- Occur in about 3 to 4 out of
1,000 births worldwide. Race and heredity do not seem to
affect the occurrence of identical twins.
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are also some very rare cases of multiple ovulation, i.e., the
release of several eggs at the same time. If these eggs are then
fertilized, they may, of course, produce multiple pregnancies.
Twins born as a result of such a pregnancy are known as
fraternal twins. If three of these children are born, they are
called triplets, four are quadruplets, five are quintuplets, and
so on. Since each of them comes from a different egg fertilized
by a different sperm cell, they may be of different sex and will
not resemble each other any more than other brothers and
sisters. |
