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Introduction

Fetal development is the process by which a fertilized egg grows into a viable fetus; this process spans approximately 38 weeks from the date of fertilization.  By convention, pregnancy dating in a clinical setting is usually referred to in terms of gestational age (time since the first day of the last menstrual period).  In contrast, the fertilization or embryonic age (time since fertilization occurred) is used to discuss the early events in embryonic development.  In patients with regular 28-day menstrual cycles, the embryonic age is approximately 2 weeks less than the gestational age.

Fetal development is divided into 3 main stages:

  • Preembryonic stage:  0-2 weeks after fertilization.
  • Embryonic stage:  2-8 weeks after fertilization.
  • Fetal stage:  >8 weeks after fertilization (or >10 weeks gestation) to birth.

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