By Kristine Ziwica for Your Baby Today

Pregnancy is indeed a scientific marvel: A cluster of cells evolves into a new life in just 280 days. But if you're an expectant parent, you're no doubt wondering, "What's going on in there?" Find the answers below with a week-by- week guide to your baby's fetal development. (Since doctors consider pregnancy a 40-week affair starting with the first day of your last period, your pregnancy really "begins" two weeks before you conceive.)

Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9 | Week 10 | Week 11 | Week 12 | Week 13 | Week 14 | Week 15 | Week 16 | Week 17 | Week 18 | Week 19 | Week 20 | Week 21 | Week 22 | Week 23 | Week 24 | Weeks 25 through 29 | Weeks 30 through 34 | Weeks 35 through 40


Week 1:

You get your period.


Week 2:

While an egg ripens in one of your two fallopian tubes, the uterus starts to form a lining of blood-rich tissue.


Week 3:

Conception. Only one of the thousands of sperm that have traveled through the fallopian tubes breaks through the egg's protective barrier to form a zygote. The cells start to divide into a small cluster as they travel down the fallopian tube towards the uterus.

Size: .006 inches


Week 4:

The tiny cluster of cells attaches itself to the uterine wall and then divides in half. One half becomes the placenta and the other half the baby. At the same time, different germ layers form. These are the building blocks for the central nervous and skeletal systems.

Size: .04 inches


Week 5:

This is a week of crucial wiring when the placenta and umbilical cord hook up to Mom. Your baby's heart -- about the size of a poppy seed -- starts beating.

Size: .05 inches


Week 6: