Title: Fetal Development: From Insemination to Birth - Pregnancy Center - Everyday Health
Headings: 
What's Going On in There During Pregnancy?
Insemination – the First Step in Pregnancy
Implantation of the Fertilized Egg
The Timeline of Pregnancy
5 Weeks Into Pregnancy: Fetal Development
8 Weeks Into Pregnancy: Fetal Development
12 Weeks Into Pregnancy: Fetal Development
16 Weeks Into Pregnancy: Fetal Development
20 Weeks Into Pregnancy: Fetal Development
24 Weeks Into Pregnancy: Fetal Development
32 Weeks Into Pregnancy: Fetal Development
36 Weeks Into Pregnancy: Fetal Development
37 Weeks to Childbirth
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Content: The blastocyst implants into the nourishing lining of the womb and becomes an embryo. The Timeline of Pregnancy
Pregnancy is measured in weeks from the first day of the mother's last period until childbirth. Pregnancy is also divided into a first trimester that includes weeks 1 through 12, a second trimester of weeks 13 through 26, and a third trimester of weeks 27 until childbirth. The process by which the cells of the embryo divide and begin to develop specific functions is called differentiation. The first trimester is considered the most critical time of a pregnancy because of the embryo's rapid differentiation into specialized cells. 5 Weeks Into Pregnancy: Fetal Development
By this time the baby's brain, spinal cord, heart, and gastrointestinal system have already begun to form, even though the embryo is only a quarter of an inch long. The embryo will start to develop arm and leg buds. The heart is beating regularly and blood is starting to move through rudimentary blood vessels. The beginnings of the structures that will become the eyes and ears are now forming.