Are you breastfeeding?

From: http://www.ivillage.co.uk/pregnancyandbaby /fertility/contraception/qas/0,,3_160250,0 0.html

Women will generally not feel like having intercourse for two to six weeks after childbirth, but of course this is variable. When a woman becomes fertile again also depends upon many variables. If she is breastfeeding exclusively – not using formula supplements at all - and she is not having periods, fertility can be delayed for up to six months with high reliability. If supplements such as formula or solid foods are offered, fertility may return. It depends upon the woman's fertility level prior to the pregnancy, the baby's patterns of feeding and whether or not periods have resumed. If a woman does not breastfeed, fertility can return as soon as two weeks after birth. More commonly, fertility returns between two and six weeks postpartum.
The bottom line is to use some form of protection all of the time unless a pregnancy would be OK. Condoms are best initially. If breastfeeding, your wife should wait until her milk supply is well established before using birth control pills.