Each woman has approximately one quarter of a million egg cells, which are kept safe inside of her ovaries throughout her lifetime. Midway through a woman's menstrual cycle, a single egg is released from a fluid-filled sac called a "follicle". At this point the egg, which is one of the largest cells in the body, is no bigger than a grain of sand. A single sperm must fertilize the egg within 24 hours or it will be absorbed, and the woman's cycle will begin again.

Sperm must overcome a variety of barriers to successfully fertilize the egg. Out of the 200-300 million sperm ejaculated, only a small number of sperm will actually reach the egg. 25% of the sperm die almost immediately upon contact with the acidic environment of the woman's vagina. This acidic environment protects the woman from infection and bacteria.

The sperm's only purpose is to fertilize an egg. They instinctively swim against the downward current of the fallopian tubes toward the egg.

Only one sperm can fertilize the egg, after which a biochemical reaction prevents any other sperm from getting inside. Once inside, the sperm loses its tail and contributes 23 of the 46 chromosomes necessary for human life. About 20 hours after fertilization, the new cell splits and is now called a “zygote”. The cells, each rapidly dividing, have little time for individual growth, which causes each new cell to be a little smaller than the previous cell. As the cluster of cells continues to divide, it becomes a hollow ball of fluid-filled cells, now called a blastocyst. The blastocyst floats down and implants itself in the nutrient rich lining of the uterus to begin receiving nutrients from the mother. By 2 weeks, the blastocyst has grown to one tenth of an inch in length. The spine is developing, and the chambers of the heart are beginning to form. Soon the heart will begin to beat.

You probably don't look like you're pregnant yet, but amazing things are happening inside of your body. The first few months are filled with activity, as your baby develops all of the internal systems she'll need.

Just four weeks after conception, before you may even know you’re pregnant, your baby’s heart has already begun to beat.

Although at this point, it's too early to tell who your baby looks like, you may be surprised at how quickly he or she is developing. Already the spinal cord is forming and the heart has begun its life's work.

At the end of the first trimester, or just three months along, your baby has arms and legs, and his fingers and toes are well underway. Eyelids, ears, ankles and wrists are already there! At this point, all major organs and systems are forming and will mature over the rest of your pregnancy.

All this development, and your baby is only 2.5 to 3 inches long and weighs about 1/2 an ounce. Pretty amazing that one so small can be causing so many changes in your body and your life!