Shortly after conception and in the first couple weeks of pregnancy, a fertilized egg travels down the fallopian tube while dividing into cells. Once it reaches the uterus, it embeds itself into the uterine wall. This is where several months of development will take place, beginning with the first trimester when the major organs and systems begin to form.
Weeks 3 to 5
At three weeks gestation, the baby's brain, spinal cord and heart are beginning to develop. Her arm and leg buds are visible and her eye and ear structures are forming. Her heart beat is steady now, and you may be able to see it as a flickering light on an ultrasound screen. By the end of the fifth week, your baby is about ¼ inch long.
Weeks 6 to 7
Between weeks six and seven of pregnancy, your baby will be about ½ inch long. His brain is continuing to form along with his lungs, jaw and nose. He has webbed formations where his fingers and toes will soon be, and all of his essential organs have begun to develop. His heart is beginning to pump blood and his hair follicles are forming.
Weeks 8 to 9
Your unborn baby has grown to be about 1 inch long. Her ears are forming inside and out, as well as her bones and eyelids. Her arms and legs are increasing in length. MayoClinic.com states that her trunk is beginning to straighten out, and her upper lip and nose are more pronounced. She may begin to move around at this stage, although you will not be able to feel it yet.
Weeks 10 to 11
By week 10 of pregnancy, your baby's neck is developing and his head is about half the length of his entire body. His genitals are developing, although it is too early to determine his gender during an ultrasound. MayoClinic.com states that he is about 2 inches long and weighs about one-third of an ounce.
Week 12
According to Family Doctor, the end of the 12th week of pregnancy marks the end of the first trimester. By this time, your baby has tripled in length and has grown to be about 3 inches long. She has fingernails and distinct facial features and will continue to grow and rapidly gain weight.
