Title: Stages of Development of the Fetus - Women's Health Issues - Merck Manuals Consumer Version
Headings: Stages of Development of the Fetus
Stages of Development of the Fetus
Fertilization
From Egg to Embryo
Development of the Blastocyst
Development of the Embryo
Placenta and Embryo at About 8 Weeks
Development of the Fetus and Placenta
Pregnancy
Stages of Pregnancy
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Content: Weeks 13 to 24
Third trimester: Weeks 25 to delivery
Events
Weeks of Pregnancy

1st Trimester
The woman’s last period before fertilization occurs. 0

Fertilization occurs. The fertilized egg (zygote) begins to develop into a hollow ball of cells called the blastocyst. 2
The blastocyst implants in the wall of uterus. The amniotic sac begins to form. 3
The area that will become the brain and spinal cord (neural tube) begins to develop. 5
The heart and major blood vessels are developing. The beating heart can be seen during ultrasonography. 6
The beginnings of arms and legs appear.