Gestational Week 4 (Conceptual Week 2):

By the end of the third week the zygote has implanted into the uterine lining, and hCG becomes detectable in maternal serum, 8 to 11 days after conception.  Bleeding from the implantation site at the uterus during this time is often mistaken for a menstrual period.

The baby, called a zygote until the end of this week, continues to divide. 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, or will have caused death of the zygote.

You may begin to experience nausea and fatigue. 

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1.Benirschke K and Kim CK. Multiple pregnancy. 2.N Engl J Med. 1973 J21;288(25):1329-36.PUBMED
2. Sandler TW. Langmans’s Medical Embryology, 7th ed.Baltimore: William and Wilkins; 1995
3. Multiple Gestations. Chitkara U and Berkowitz RL In Gabbe: Obstetrics - Normal and Problem Pregnancies, 4th ed. 2002
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. 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