Hello folks. Swedishguy, while I'm no expert, the fact that your wife's family is bringing in fish over 5kg should be of some concern. From my understanding, most of the pollution in Dianchi is due to, as hedgepig says, the eutrophication that came from both fertilizer runoff and from kunming's wastewater treatment facilities. However, in a lake that large, this close to an urban area, there are likely to be heavy industrial pollutants in the water as well.
The larger the fish you're eating out of the lake, the more bio-accumulation has occurred of non-filtered chemicals such as mercury, lead, and other nasty things you want nowhere near your baby. One of the reasons why pregnant women are advised against eating tuna is that, as a larger fish, mercury bioaccumulates in tuna at a much higher rate. It'll probably be better for the baby to lay off the Dianchi fish.