If you haven't already, quit using all illegal drugs, alcohol and tobacco. Need help quitting? Get in immediate contact with your doctor. These substances are closely linked to birth defects, pre-term labor and miscarriage. Any prescription medications you're taking should also be run by your obstetrician as soon as possible; many legitimate meds can have an adverse effect on your fetus and its development.
Ideally, you already have an established relationship with a trustworthy OB/GYN or midwife. If not, now's the time to get recommendations from friends or set up informational interviews. You'll be spending an absurd amount of time with this person(s) over the next year, so be sure to find someone you respect and are comfortable divulging your most embarrassing secrets to.
Also, you should be taking a daily prenatal vitamin to help both you and the baby through the gestational period. Folic acid is particularly critical to fetal neural tube development, so be sure that whatever brand you choose contains at least 400 micrograms, according to the American Pregnancy Association.