The following information is found at the CDC site:

Not all birth defects can be prevented. But a woman can increase her own chances of having a healthy baby by managing health conditions and adopting healthy behaviors before becoming pregnant. This is important because many birth defects happen very early during pregnancy, sometimes before a woman even knows she is pregnant.

Here are 10 steps a woman can take to get ready for a healthy pregnancy:

1. Take 400 micrograms (mcg) of folic acid every day. Folic acid is a B vitamin. If a woman has enough folic acid in her body at least 1 month before and during pregnancy, it can help prevent major birth defects of the baby’s brain and spine.
   
   
2. Don't drink alcohol at any time during pregnancy. When a woman drinks alcohol, so does her unborn baby. Alcohol in the woman’s blood passes through the placenta to her baby through the umbilical cord. There is no known safe amount of alcohol to drink while pregnant. There also is no safe time during pregnancy to drink and no safe kind of alcohol. Drinking alcohol during pregnancy can cause a baby to be born with a birth defect.
   
   
3. Don’t smoke. The dangers of smoking during pregnancy include premature birth, certain birth defects (cleft lip or cleft palate), and infant death. Even being around cigarette smoke puts a woman and her unborn baby at risk for problems. Quitting smoking before getting pregnant is best. But for a woman who is already pregnant, quitting as early as possible can still help protect against some health problems for the baby, such as low birth weight. It’s never too late to quit smoking.
   
   
4. Don’t use “street” drugs. A woman who uses illegal—or “street”—drugs during pregnancy can have a baby who is born premature with low birth weight, or has other health problems, such as birth defects. A woman who uses cocaine while pregnant is more likely to have a baby with birth defects of the arms, legs, urinary system, and heart. Other drugs, such as marijuana and ecstasy, also can cause birth defects among babies.
   
   It also is important that a woman not use "street" drugs after she gives birth, because such drugs can be passed through breast milk to her baby and can affect the baby’s growth and development. If you use "street" drugs, talk with your doctor about quitting before you get pregnant.
   
   
5. Talk to a health care provider about taking any medications. Taking certain medications during pregnancy can cause serious birth defects, but the safety of many medications taken by pregnant women has been difficult to determine. If you are pregnant or planning a pregnancy, you should not stop taking medications you need or begin taking new medications without first talking with your doctor. This includes prescription and over-the-counter medications, as well as dietary or herbal products.
   
   
6. Prevent infections. Some infections that a woman can get during pregnancy can be harmful to the unborn baby. Learn how to help prevent infections.
   
   
7. Talk to your doctor about vaccinations (shots). Many vaccinations are safe and recommended during pregnancy, but some are not. Having the right vaccinations at the right time can help keep a woman and her baby healthy.
   
   
8. Keep diabetes under control. Poor control of diabetes during pregnancy increases the chances for birth defects and other problems for the baby. It can also cause serious complications for the woman. Proper healthcare before and during pregnancy can help prevent birth defects and other poor outcomes.
   
   
9. Reach and maintain a healthy weight. A woman who is obese (a body mass index of 30 or higher) before pregnancy is at a higher risk for complications during pregnancy. Obesity in the woman also increases the risk of several serious birth defects for the baby. If you are overweight or obese, talk with your doctor about ways to reach a healthy weight before you get pregnant.
   
   
10. See a health care professional regularly. A woman should be sure to see her doctor when planning a pregnancy and start prenatal care as soon as she thinks that she is pregnant. It is important to see the doctor regularly throughout pregnancy, so a woman should keep all her prenatal care appointments.

Six tips from the American Pregnancy Association:

Awareness and education are the first steps to preventing birth defects. The immediate step following awareness and education is taking action. There are a number of things you can do to increase the probability of having a healthy pregnancy and a healthy baby. Some are more challenging than others because they require that you break habits, but it is worth your effort.

Here are a variety of tips you can use to prevent birth defects as you contemplate starting or adding to your family:

Tip 1. The first and foremost tip is maintaining preconception health; eating well balanced and nutritional meals, and taking a multivitamin daily that includes the recommended 400 mcg of folic acid.

Tip 2. If you are sexually active and pregnancy is a possibility, make sure you take a multivitamin daily which includes the recommended 400 mcg of folic acid and other essential B vitamins.

Tip 3. Avoid all activities that could potentially lead to birth defects including alcohol, tobacco, illicit drugs, and caffeine.

Tip 4. Seek an annual gynecological and wellness exam.

Tip 5. Obtain genetic counseling and birth defect screening, particularly if you have any family history of birth defects or if you are 35 years of age or older.

Tip 6. Help your family or friends who might be considering parenthood by informing them that January is Birth Defects Prevention Month. You can send an e-mail and link to this page to everyone in your address book.”

More recommendations from the CDC:

• Premature Birth: Important growth and development occur throughout pregnancy – all the way through the final months and weeks. Babies born three or more weeks earlier than their due date have greater risk of serious disability or even death. Learn the warning signs and how to prevent a premature birth.

• Folic Acid: Folic acid is a B vitamin that can help prevent major birth defects. Take a vitamin with 400 micrograms (mcg) of folic acid every day, before and during pregnancy.

• Smoking during pregnancy is the single most preventable cause of illness and death among mothers and infants. Learn more about the dangers of smoking and find help to quit.
  
  
• Alcohol: When you drink alcohol, so does your unborn baby. There is no known safe amount of alcohol to drink while pregnant.
  
  
• Vaccinations: Talk to your doctor about vaccinations (shots). Many are safe and recommended during pregnancy, but some are not. Having the right vaccinations at the right time can help keep you and your baby healthy.
  
  
• Flu and Pregnancy: If you're pregnant, a flu shot is your best protection against serious illness from the flu. A flu shot can protect pregnant women, their unborn babies, and even their babies after birth.
  
  
• Infections: You won’t always know if you have an infection—sometimes you won’t even feel sick. Learn how to help prevent infections that could harm your unborn baby.
  
  
• HIV: If you are pregnant or are thinking about becoming pregnant, get a test for HIV as soon as possible and encourage your partner to get tested as well. If you have HIV and you are pregnant, there is a lot you can do to keep yourself healthy and not give HIV to your baby.
  
  
• West Nile Virus: Take steps to reduce your risk for West Nile virus and other mosquito-borne infections.
  
  
• Diabetes: Poor control of diabetes during pregnancy increases the chance for birth defects and other problems for your baby. It can cause serious complications for you, too.
  
  
• High Blood Pressure: Existing high blood pressure can increase your risk of problems during pregnancy.
  
  
• Medications: Taking certain medications during pregnancy might cause serious birth defects for your baby. Talk to your doctor or pharmacist about any medications you are taking. These include prescription and over-the-counter medications and dietary or herbal supplements.
  
  
• Environmental and Workplace Exposures: Some workplace hazards can affect the health of your unborn baby. Learn how to prevent certain workplace hazards. If you are worried about a specific substance, please click here.
  
  
• Unborn Babies Exposed to Radiation: If you think you might have been exposed to radiation, talk with your doctor.
  
  
• Pediatric Environmental Health Specialty Units: 
  A network of experts in children's environmental health.