I can attest that NFP works very well. And thank God it does because seven weeks postpartum my fertility returned, and it has been a very hard birth. We were able to postpone pregnancy and allow me to recover.
I do think it is Biblically permissable to regulate birth. God gave us simple indications of fertility and a natural rise and fall in fertility. But you always have to love the whole person, including their God given ability to bring life into this world.
"seven weeks postpartum my fertility returned, and it has been a very hard birth"
Wow! Were you nursing your child or using formula?