Weeks 1 and 2: Getting ready
It might seem strange, but you’re not actually pregnant the first 2 weeks of the total 40 weeks that we count. Day 1 of the 40 week period is the 1st day of your period. Conception (official pregnancy) typically occurs about two weeks after your period begins.
Week 3: Fertilization
The sperm and egg unite in one of your fallopian tubes to form a one-celled entity called a zygote. While this might be a bit too much information, this is the true beginning of your baby. This zygote starts to multiply and take on the form of something that looks like a raspberry. It moves toward your uterus where it will spend the next 37 weeks.
Week 4: Implantation
By the time it reaches the uterus, the rapidly dividing ball of cells — now known as a blastocyst — has separated into two sections. The inner group of cells will become the embryo. The outer group of cells will become the membranes that nourish and protect it. On contact, it will burrow into the uterine wall for nourishment. This process is called implantation. The placenta, which will nourish your baby throughout the pregnancy, also begins to form.
Week 5: Embryo Development
The fifth week of pregnancy, your baby (an “embryo”) is now made of three layers. These layers over the next 2 months begin to form different organs (for example the lungs and heart) within your baby. By the end of this week, your baby is likely about the size of the tip of a pen.
Week 6: A small heart is beating
Small facial features will begin to appear and small buds will soon become arms and legs.
Week 7: Baby’s face and brain are developing
Seven weeks into your pregnancy baby’s brain and face are rapidly developing. It’s little nose is starting to develop and so are its eyes. By the end of this week, your baby might be a little bigger than the top of a pencil eraser.
Week 8: Baby’s eyes are visible
Eight weeks into your pregnancy your baby’s arms and legs are growing longer. The upper lip and nose have formed. The trunk of your baby’s body is beginning to straighten. By the end of this week, your baby might be about 1/2 inch long.
Week 9: Baby’s fingers and toes form
In the ninth week of pregnancy your baby’s arms grow, develop bones and bend at the elbows. Toes form, and your baby’s eyelids and ears continue developing. By the end of this week, your baby might be about 3/4 inch long.
Week 10: Baby’s head and neck are developing
Week 11: Baby is a boy or a girl!
Your baby is now officially described as a fetus and is now a boy or a girl. It’s unfortunate that we have to wait another 9 weeks or so until we can tell by ultrasound. By now your baby might measure about 2 inches long weigh almost 1/3 ounce.
Week 12: Baby’s has fingernails!
Baby is developing fingernails and now has a human profile. At this point, he or she might be about 2 1/2 inches long and weigh about 1/2 ounce.
Kudos to the Mayo Clinic for having an excellent summary as well. http://www.mayoclinic.com/health/prenatal-care/PR00112