Don"t show up to your 6 week postpartum check up pregnant.
Updated November 27, 2014.
Question: Can I get pregnant shortly after giving birth?
Answer: The answer is a resounding yes! Many moms are lead to believe that they cannot get pregnant soon after delivery. Whether you had a vaginal birth or cesarean section, your body is capable of getting pregnant very shortly after giving birth. You may not even notice that you have had your period because you can ovulate before having your first period.
Despite the fact that it is not recommended that you have sex prior to your six week check up, it happens. If you don't use birth control you can get pregnant. Breastfeeding, while it may delay ovulation in some women, is not an effective method of birth control unless you are following the Lactational Amenorrhea Method (LAM).
If you think you are pregnant, talk to your practitioner.
Source...