The First Two Weeks
Shortly after a woman’s period begins, her body begins preparing for the possibility of pregnancy.
Approximately 2 weeks into her cycle, a woman releases an egg from one of her ovaries into her adjacent fallopian tube. Conception is now possible for the next 24 hours or so 8 and signifies the beginning of pregnancy.9
The single-cell embryo has a diameter of approximately 4 thousandths of an inch.10
The cells of the embryo repeatedly divide as the embryo moves through the Fallopian tube into the woman’s uterus or womb. Implantation, the process whereby the embryo embeds itself into the wall of the womb, begins by the end of the third week and is completed during the fourth week of pregnancy.11
The 4-week embryo is less than 1/100th of an inch long.
4 to 6 Weeks
By 5 weeks, development of the brain, spinal cord,12 and heart13 is well underway.
The heart begins beating at 5 weeks and one day14 and is visible by ultrasound almost immediately.15
By 6 weeks, the heart is pumping the embryo’s own blood to his or her brain and body.16 All four chambers of the heart are present17 and more than 1 million heartbeats have occurred.18 The head, as well as the chest and abdominal cavities have formed19 and the beginnings of the arms and legs are easily seen.20
The 6-week embryo measures less than ¼ of an inch long from head to rump.
6 to 8 Weeks
Rapid brain development continues with the appearance of the cerebral hemispheres at 6½ weeks.21
The embryo reflexively turns away in response to light touch on the face at 7½ weeks.22
Fingers are beginning to form on the hand.23
By 8 weeks the developing human measures about a ½ inch from head to rump. “