I actually don't know. The periods for ritual purification and confinement postpartum are in Leviticus, but that might be one of the bits of Leviticus that fundies think doesn't apply to them and don't bother with it.
Do fundies observe Candlemas? The Feast of Purification? If so, he would know that this holy day commemorates the presentation of Our Lady in the temple after she had observed the rites relating to postpartum purification. Even in comparatively recent times, women were still "churched". I was called for special blessing each time I reappeared postpartum, so the tradition still continues in some form.
The thing is, if Nathan as a fundie knows about any of that stuff, he knows the reason behind it is that the Lord commanded special treatment for women for a period after birth. The special treatment involved them staying in the fucking house, i.e. resting. And also not to get any dick. (I don't trust Nathan to have had the good sense to understand why he must leave Nurie alone for some weeks, although I am reluctant to think he would actually hurt her. But still, I'm not sure he will understand why the full six weeks is important.)
Like most early religious commandments - especially those in the Old Testament - this is a public health issue. If women were ritually unclean, they stayed home, husbands did not interfere with them, they were generally only with women and those close to them, they weren't out in the streets or working. This was to let them rest and get better. (It's not a sin issue because no sin attaches in the OT or the texts derived from it to the act of childbirth itself. It's just made clear that until after a passage of about four to eight weeks, the woman's activities are to be restricted, and the strongest prohibition that could be imposed was ritual uncleanliness.)
I know this is an autistic tangent, but well, fundies are religiously autistic.