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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Reviewed by Mark Curran, M.D. FACOG

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