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) |
|
|
|
|
|
|
|
|
|
|
| CNS* |
|
|
| Heart |
|
|
|
|
|
| Ear |
|
|
|
| Eyes |
|
|
|
| Limbs |
|
|
|
|
|
|
| Lip
|
|
|
|
|
|
|
|
|
| Palate |
|
|
|
|
|
|
|
| Teeth |
|
|
|
|
|
| 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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REFERENCES
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
