Dr. Phil Boyle (Mary Ann's brother) uses LDN in women who would like to become pregnant but have had miscarriages and a very irregular cycle. He says, that he lets them stay on LDN until about 10-12 weeks of pregnancy. There have been women on 50mg Naltrexone, because of drug addiction, and they stayed on it throughout their pregnancy and the only drawback was that the babies were a bit lighter in weight.
You can listen to his whole audio from the 2nd annual LDN conference.
In the end he takes questions from the audience, and the woman next to me asked about pregnancy and LDN.
I assume, that because he is a fertility expert, and has had a few successfull pregnancies in his clinic with this regiment, it is fairly save. After those initial 10 - 12 weeks the body is producing higher levels of hormons anyhow and that should keep the MS at bay until after delivery. I have not heard any specifics about LDN and nursing, but I would imagine, that that would be even less of a risk.
But everybody should form their own opinion!