Fetal Development : Week 5
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Gestational Week 5 (Conceptual Week 3):

Week 5 is the beginning of the embryonic period which lasts from the the 5th to the 10th week. During the embryonic period the major organs will be formed from three layers of cells (germ layers). It is during this critical period that many birth defects occur in the developing embryo. Most of these birth defects will have no known cause or be due to a combination of factors (multifactorial). 

By the end of the week the heart is pumping blood. Although the beating heart is usually not seen this early, the embryo and or yolk sac may be seen on sonogram this week.

Although less than 5 percent of birth defects may be attributed to external substances or maternal conditions, it is best to avoid alcohol and unnecessary medications.  It is also best to continue to take medications that have been prescribed for you such as thyroid hormone, antibiotics, and insulin or other medications for diabetes, since poor control of maternal disease may have an unfavorable effect on your pregnancy. Most health care providers advise waiting until after the first trimester to use hair dyes.

You may experience breast tenderness at this point in the pregnancy. As the pregnancy progresses you may expect the areolae to enlarge and become more deeply pigmented.

You may also begin to experience worsening nausea and vomiting.  Nausea and vomiting of pregnancy are thought to be caused by high levels of estrogen and human chorionic gonadotropin (HCG) (hormones that help maintain the pregnancy). Up to 80 percent of women will eventually complain of nausea and or vomiting during their pregnancies. Fortunately, these symptoms resolve in most women by 20 weeks.  Some measures may help reduce your nausea in the meantime. See "Tips for Reducing Nausea" . A more severe form of nausea and vomiting during pregnancy, called hyperemesis gravidarum, can lead to serious dehydration and derangements in components of the blood. Hospitalization is often required to treat dehydration and evaluate these patients for other possible causes of their symptoms .

Bleeding at this point during the pregnancy may be a sign of an early miscarriage, a tubal pregnancy, or bleeding from the placenta, and should be reported to your health care provider at once.

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.Moore KL, Persaud TVN, The developing human: clinically oriented embryology, 7th edition, Saunders, 2003:520.
2. Sandler TW. Langmans’s Medical Embryology, 7th ed.Baltimore:  William and Wilkins; 1995
3. Hadlock FP, et al. In utero analysis of fetal growth: a sonographic weight standard.Radiology. 1991;181:129-33.PUBMED
4. 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
5. Quinla JD and Hill DA. Nausea and vomiting of pregnancy. Am Fam Physician. 2003 1;68(1):121-8. PUBMED

Reviewed by Mark Curran, M.D. FACOG

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