We never practiced NFP and I requested that she stop taking birth control, about a year ago as I entered into RCIA. I have mentioned NFP but she is not that that interested.
Yes, I can understand that. Especially if she is not familiar with any new method (she may be thinking of just a calendar/rthym type method). That is why I suggested you research to see when a class will be held in your area, sign up for it and go together. Our teaching couple from CCLI (
ccli.org) talked quite a bit about the husband’s role in NFP. With the sympto-thermal method, you can take on most of the work of the method–you take her temp, you do the charting, you interpret the rules, and you do the abstaining (in that you don’t pressure her during her fertile times).
As for dissuading her from taking the pill, there are many arguements for that and you know your wife best to see which will be strongest for her. You can talk about the serious health risks (blood clots, etc.); the risk to her future fertility for when you are ready to add to your family; and the risks to your current baby from hormones leaching into her breastmilk; and finally, the environmental issues from all the excess estrogen in our water.
Also, she needs to know that the pill is not 100% effective (no birth control method is). It is safer to rely on your own abstinence than on any other method.
And again, I will say that at just 6 weeks post-partum, she’s likey very emotional and not really ready to make a big decision. She’s already been pressured by the ob-gyn to start taking the pills (with probably lots of scary talk about babies 9 months apart!

), she won’t appreciate feeling pressured by you too, so proceed in love. Be gentle, but consistent.
Also, her culpability for the sin of taking the pill is lessened by how much pressure she did feel. Since you do not want to be a party to this sin and because you want every reason for her to get off the pill, you should consider not initiating marital union while she’s on them. Hopefully you’ll be able to get into an NFP class quickly and start using that method! And of course, continue to pray for her every day.