The zygote begins to develop rapidly within a day after it is fertilized. In 3 days, a cluster of cells (morula) will exit the Fallopian tube and enter the uterus. Then a fluid-filled cavity will form in the center of the growing morula. The fertilized egg -- now called a blastocyst -- starts dividing into hundreds of cells by the eighth day of post-fertilization. Once safely in the uterus, it embeds itself within the uterine lining.

Pregnancy is different for every woman, especially in the first trimester. Some women feel physically out of sorts, while others have emotional ups and downs (as if they were premenstrual). For others, the only sign of having conceived is the absence of menstruation.

So if you think you may be pregnant, be on the lookout for one or more of these telltale signs:

• Tender, swollen breasts
• Nausea, queasiness
• Increased urination
• Tiredness
• Food cravings
• Darkening of the skin around the nipples (areolas)

On the emotional end, you may feel weepy, unstable, irrational, and all-around irritable. If you're more prone to mood swings, you may feel joy and elation on a good day and, on a less-than-cheery one, misgivings or fear. These feelings may set in as early as the first month of pregnancy, or they may develop sometime in the second. Or, they may not affect you at all.

Note: If you have any bleeding, pelvic pain, non-stop vomiting (unable to even tolerate liquids), painful urination, or sudden swelling of your hands, feet, or face, call your doctor right away.

Planning on being pregnant or hoping you already are? If so, you need to eat accordingly. That means getting the proper amount of the correct foods.

If you're trying to conceive or think you may be pregnant, you should start taking a prenatal vitamin at least 3 months before you plan to conceive. Choose a vitamin with at least 400 mcg of folic acid, a B vitamin that's been shown to reduce the risk for severe birth defects. If you haven't already started taking prenatal vitamins with folic acid, you should begin doing so right away.