Fetal Development : Week 4
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Gestational Week 4 (Conceptual Week 2):

By the end of the third week the baby, called a zygote until the end of this week, has implanted into the uterine lining. Bleeding from the implantation site at the uterus during this time is often mistaken for a menstrual period.

Human chorionic gonadotropin (the pregnancy hormone) first becomes detectable in the mother's blood and urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age).

Rarely the zygote may separate into two individual embryos  8 to 13 days after fertilization resulting in identical twins with one placenta and one amniotic sac. Even less commonly conjoined twins may be formed if the zygote separates at 13 to 15 days after fertilization. Conjoined twins are seen in about 1 per 50,000 deliveries.

Until this point in development toxic exposures  will have had no effect on the zygote , or will have caused death of the zygote.

You may begin to experience nausea and fatigue. 

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REFERENCES


1. Sandler TW. Langmans’s Medical Embryology, 7th ed.Baltimore: William and Wilkins; 1995
2. Hay DL, Lopata A.Chorionic gonadotropin secretion by human embryos in vitro. J Clin Endocrinol Metab. 1988 Dec;67(6):1322-4.PMID: 2461389
3. Wilcox AJ, et al. Time of implantation of the conceptus and loss of pregnancy.N Engl J Med. 1999 Jun 10;340(23):1796-9.PMID: 10362823
4. Lohstroh P, et al. Daily immunoactive and bioactive human chorionic gonadotropin profiles in periimplantation urine samples. Biol Reprod. 2006 Jul;75(1):24-33. Epub 2006 Mar 8.PMID: 165250351.Benirschke K and Kim CK. Multiple pregnancy. 2.N Engl J Med. 1973 J21;288(25):1329-36.PUBMED
5. Multiple Gestations. Chitkara U and Berkowitz RL In Gabbe: Obstetrics - Normal and Problem Pregnancies, 4th ed. 2002
6. Moore KL, Persaud TVN, The developing human: clinically oriented embryology, 7th edition, Saunders, 2003:520.
7. Brent RL. The effect of embryonic and fetal exposure to x-ray, microwaves, and ultrasound: counseling the pregnant and nonpregnant patient about these risks. Semin Oncol 1989;16:347–68.

Reviewed by Mark Curran, M.D. FACOG

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