A woman in her post-natal period can get pregnant as soon as she ovulates which can be anywhere from 4 weeks p-n on. Since you don't know when you're ovulating after pregnancy and it can occur at any time, it's best to use birth control right away.
If you're breastfeeding, there's no guarantee that ovulation will be suspended, and I would recommend a birth control safe for breastfeeding mothers or condoms.
If you're not breastfeeding, you should resume your period about 7-9 weeks after the birth of your baby, but these numbers aren't solid.
If you're TTC, it's usually recommended that you wait a year after a vaginal delivery and about 18 months after a c-section to reduce the risk of complications of the pregnancy including miscarriage.
at 1:03 AM on May. 27, 2009