Gestational Week 3
(Conceptual Week 1):
During the third week following the last menstrual
period, you may begin to experience fatigue and swollen or tender breasts as
the first signs of pregnancy. It is during the third week fertilization occurs, the sex of the baby is determined , and
twins may be formed.
The released egg and the father's sperm usually meet
in the upper part of the Fallopian tube. Once the sperm penetrates the
egg, the genetic material from the sperm and egg merge to form a
complete set of 23
chromosomes.
The process of fertilization is now
complete, and the gender of the fertilized egg is determined by the
father's contribution of either an X or Y chromosome. The fertilized egg
is called a zygote. Within days of fertilization the zygote
begins to produce
human chorionic gonadotropin (hCG)
"the pregnancy
hormone" , which will
maintain the corpus luteum until the placenta is able to produce enough
progesterone to support the pregnancy.
The zygote continues to divide as it travels for 3 to 7 days
down the Fallopian tube to the uterus. If the zygote separates into
two separate embryos within 4 days after fertilization identical
twins with separate placentas and separate amniotic sacs will occur.
Identical twins with one placenta and separate amniotic sacs will
occur if the zygote divides 4 to 8 day after fertilization [1].These
twins are referred to as
monozygotic
or "identical" twins. Identical twins account for about 30% of naturally
occurring twins in the United States [2]
|
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| If two eggs
are released each egg may be fertilized by separate sperm to
form two zygotes. Each zygote implants in the uterus individually
and develops its own membranes and placenta. The two zygotes
continue to develop as two separate embryos. These twins are
referred to as
dizygotic (commonly known as fraternal) twins. Fraternal
twins account for about 70 % of naturally occurring twins in the
United States [2]. |
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