The egg is fertilized and cellular development begins immediately. The gender of the baby can be determined before the egg is implanted in the womb.

When the new life implants, it is already composed of hundreds of cells. It has developed a protective hormone to prevent its rejection as foreign tissue in the mother’s body.

At 17 days blood cells have developed.

At 18 days the muscle that will become the heat begins to pulsate.

At 19 days the eyes begin to develop.

At 20 days, the foundation of the brain, spinal cord and e
ntire nervous system has been laid.

At 21 days the heart begins to beat.

At 28 days the backbone and 40 pairs of muscles are developing along the trunk. Arms and legs are forming.

Photo credit: http://www.babycenter.com/fetal-development-images-4-weeks