How Pregnancy Works

First Trimester: From Conception to Embryo

The fertilized egg makes the journey through the Fallopian tube to the uterus, where it will implant.
The fertilized egg makes the journey through the Fallopian tube to the uterus, where it will implant.

A man releases millions of sperm with each ejaculation. Once inside the woman's body, the sperm make a mad dash to find the egg. If one sperm reaches the egg and penetrates it, fertilization occurs.

Some women know immediately that they're pregnant -- call it a new mother's intuition. However, the only way to know for sure is to take a pregnancy test. All pregnancy tests look for the same thing: the presence of human chorionic gonadotropin (hCG), a hormone produced only during pregnancy.

Home pregnancy tests measure hCG in urine, while a test in the doctor's office will identify hCG in a blood sample The blood test can pick up very tiny amounts of the hormone and identify pregnancy earlier than a home pregnancy test. Still, most home tests are 97 to 99 percent accurate if taken correctly.

Then things start progressing rapidly inside the mother's womb. The sperm and egg merge to form a little single-celled organism called a zygote, which consists of the 23 chromosomes from the man's sperm and the 23 chromosomes from the female's egg. These chromosomes will determine the baby's hair color, eye color and whether the baby will be a boy or a girl.

Soon after fertilization, the zygote makes the trip through the Fallopian tubes to the uterus. During this journey, the zygote divides. Within 72 hours, it will have gone from one cell to eight cells.

Remember the zygote? This little ball of cells divides until it contains about 100 cells. Then it becomes known as a blastocyst. The inner group of cells will form the embryo. The outer group of cells forms the placenta, which will provide nourishment.

Three weeks into the pregnancy, the blastocyst implants itself into the mother's uterine wall and releases hCG. This occurs only a few days after conception. Her doctor will begin counting the 40 weeks of pregnancy from the start of her last period, although conception normally occurs about two weeks after that.

By the fifth week of the pregnancy, the brain, spinal cord, heart, and other organs begin to form. The embryo is now made up of three layers: the ectoderm, mesoderm, and endoderm. Every organ and tissue will develop out of these three layers. The ectoderm will form the nervous system and backbone; the mesoderm will form the heart and circulatory system; and the endoderm will form the lungs, gastrointestinal tract, thyroid, liver and pancreas. The placenta has already begun to form, as well as the umbilical cord, which will deliver nutrients to -- and remove wastes from -- the growing embryo.

Next, we'll look at the rest of the first trimester.