The 40% statistic came from two studies in 1991 [PDF] and 1992 [PDF] from self reported data.The general take is that since it's self-reported that it is likely much higher (since people tend to lie about things like this). However, that data is also over 30 years old, and things could have changed since then. There's no direct data on whether it's getting better or worse, but if I may speculate a little here, I would suggest there's probably a correlation between private domestic abuse and public incidents of police violence, brutality, and killings over time.

The only data I could find on domestic abuse and the nursing profession is actually about abuse against the nurses. Notably, it was still self-reported data, from a single hospital in India (where the baseline for domestic violence is higher than in North America or Western Europe).

Unfortunately, I don't think there's a lot of good data out there. It's an inherently difficult topic to study, and even though it's important, it's unlikely that any study will produce actionable results. The limited amount of funding tends to go towards studies that can improve the world (and/or someone's pocketbook), rather than report on how it is.