Fetal Development : Week 6
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Gestational Week 6 (Conceptual Week 4):

The embryo is now about the size of a pea. This week the brain and spinal cord are formed from a cylinder shaped structure, the neural tube. The eyes, nostrils , and arms also begin to take shape. Progesterone production by the corpus luteum declines this week as the placenta begins to increase its production of progesterone.

Your health care provider may notice that your vaginal tissue and cervix appear to have a bluish discoloration (Chadwick's sign). They may also notice softening of the lower uterus above the cervix (Hegar's sign). The baby's heartbeat may sometimes be seen by using a transvaginal sonogram at this time.

Mood swings are especially common from the 6th to the 10th week and are believed to be caused by increasing levels of progesterone and estrogen.

Table 1. Gestational Age When Organ Systems are Most Sensitive to Birth Defects.

Developmental stage Embryonic Stage Fetal Stage
Gestational  Age  (Months)                               1                                2                             3
Gestational  Age (Weeks) 1 2 3 4 5 6 7 8 9 10 11 12 13
Postconception Age  (weeks) 0 0 1 2 3 4 5 6 7 8 9 10 11
Developing Organ(s)  
External genitals    
Weight ounces 1.3 1.6 2 2.6
grams 35 45 58 73
Length inches 0.16 0.39 0.63 0.91 1.22 1.61 2.13 2.64
centimeters 0.4 1 1.6 2.3 3.1 4.1 5.4 6.7

The red bars in the table  show the gestational age when different organ systems are most sensitive to major birth defects in that organ system. The gray bars show the gestational age when different organ systems are sensitive to functional defects and minor malformations.

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1. Chervenak FA and Gabbe SG. Obstetric Ultrasound: Assessment of Fetal Growth and Anatomy   In Gabbe SG et al ed: Obstetrics - Normal and Problem Pregnancies, 4th ed. New York.: Churchill Livingstone 2002
2. Liu JH. Endocrinology of pregnancy. In Maternal-fetal Medicine: principles and practice. Resnik R, Creasy K, Iams JD ed. 5th ed Philadelphia: W.B. Saunders Company 2004
3. Moore KL, Persaud TVN, The developing human: clinically oriented embryology, 7th edition, Saunders, 2003:520.
4. Sandler TW. Langmans’s Medical Embryology, 7th ed.Baltimore:  William and Wilkins; 1995
5. Hadlock FP, et al. In utero analysis of fetal growth: a sonographic weight standard.Radiology. 1991;181:129-33.PUBMED
6.  Hadlock FP, et al Fetal crown-rump length: reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US.Radiology. 1992;182(2):501-5. PUBMED

Reviewed by Mark Curran, M.D. FACOG

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