During fertilization, the sperm undertake a long and perilous journey into the woman's body. Their quest is to cross the cervix before they die. Out of millions of sperm, only one will fertilize the egg. Once that one sperm crosses the threshold, an entirely new journey begins.

Photo courtesy Anita Patterson/MorgueFile After roughly 40 weeks, a tiny mass of cells transforms into a newborn. See more pregnancy pictures.

For the next 40 weeks or so, the mother's body will undergo a series of transformations as it nourishes and houses a new life. The egg will also transform -- from a tiny mass of cells into a living, breathing baby.

In this article, we'll look inside the womb to learn how the fetus develops during each trimester of the pregnancy, see how the baby finally makes its way into the world, and find out what could go wrong during pregnancy.

InfertilityAbout 10 percent of reproductive-age couples in the U.S. suffer from infertility, which occurs when one or both partners has a problem with their reproductive system that makes conception difficult or impossible [American Society for Reproductive Medicine]. A couple is said to be infertile when they have had sexual intercourse without using birth control for more than a year without conceiving. In about 40 percent of infertility cases, the problem lies with the male partner. In another 40 percent, the problem lies with the female, and the remaining 20 percent are caused by either a combination of both partners or some unexplained problem [The National Infertility Association]. Fortunately, there are several methods for treating infertility, including in-vitro fertilization and medication.

Starting a family is a big decision -- planning ahead is important. Health experts recommend that women who are trying to conceive see their doctor or Ob/Gyn (obstetrician/gynecologist) for a pre-pregnancy examination, to make sure that their body is up to the task.

During a typical exam, the doctor will:

• Take medical history to determine whether the woman has a history of any disease (such as diabetes or high blood pressure) that might be dangerous during the pregnancy
• Get family history and peform genetic testing to identify any hereditary disorders
• Perform a health screening, including an examination for infections such as herpes and other STDs or urinary tract infections
• Check to make sure the mother-to-be is up to date on all of her immunizations, especially chicken pox and rubella, which can cause miscarriage or birth defects

Women who are trying to conceive should watch their diet and take daily prenatal vitamins. Taking at least 400 micrograms of folic acid each day can significantly protect against birth defects. They should also stop smoking, because it can lead to low birth weight and other complications.

Next, let's review what happens during conception and the first trimester.