Fetal Development : Week 3
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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 that 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]
 

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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REFERENCES

1.Benirschke K and Kim CK. Multiple pregnancy. 2.N Engl J Med. 1973 J21;288(25):1329-36.PUBMED
2. MacGillivray I.Epidemiology of twin pregnancy.Semin Perinatol.1986;10:4-8.PUBMED

Reviewed by Mark Curran, M.D. FACOG

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