A typical pregnancy is nine months long, but to give your baby a healthy start, think of it as twelve months -- including the three months before you get pregnant. This means that when you start thinking about trying to conceive, you should:

• See your doctor for a pre pregnancy checkup. Don't forget to ask about things like family medical history, risk of birth defects, genetic conditions, and chronic illnesses. Discuss all the medications you take and make sure they're safe during pregnancy.
• In addition to eating a healthy diet (lots of leafy greens, lean proteins, and fiber), boost your nutrients with a multivitamin specially formulated for pregnancy -- usually called a prenatal vitamin. It's particularly important to get sufficient folic acid before getting pregnant. This nutrient helps prevent birth defects like spina bifida; since many of these conditions arise very early in pregnancy, you need healthy levels of folic acid right from the start. Look for a multivitamin that contains 400 micrograms of calcium. Ask your doctor or midwife to recommend a vitamin for you. 
• If you smoke, quit. Smoking poses a host of risks to a developing baby, including birth defects and low birth weight. It also doubles your risk of having an ectopic pregnancy. You may also find it more difficult to become pregnant in the first place if you smoke, as smoking is strongly linked with infertility in both women and men.
• Get checked for hepatitis B and C, sexually transmitted infections, and HIV.
• Get any health problems -- like diabetes and high blood pressure -- under control. If you are seriously overweight, talk to your doctor about how to maintain a healthy weight.