(1) A lot of stillbirths. Stillbirths aren't unknown in genlias, but they are (and I have no explanation for this, either) most common in three circumstances: (a) mother's first birth (b) one of a pair of twins (c) mother's last birth. A string of non-twin stillbirths from a mother who survives is wildly bizarre compared to the pattern I have been seeing. (It's pretty obvious why a mother's last birth might be stillborn; that's a risk associated with mother being in her forties. I would imagine the first-birth stillbirth is often attributable to difficulties of a first labor.)
(2) An occasional living child in a string of stillbirths (or living children), who then die (perhaps several at a shot) during the summer after they are over a full year old. Dying during the winter I'd be happy to chalk up to the flu (or, altho again, not likely in the Netherlands, TB). Not in the summer. I'd believe dying of gastro stuff in the summer, but not in an agricultural setting in the Netherlands in the 19th century.
(3) A mother who survives this rather unpleasant life experience to die at around age 70 or later -- and a husband who often predeceases substantially. Again, wildly unexpected.
Not likely to be rickets in a Dutch population in the 19th century, especially not an agricultural family. They ate well (and I would expect this particular group to be getting lots of fish). The women spent time outdoors. The Dutch have understood for a long time the importance of sunshine to good health. Not likely to be genetic or something like Rh, given that the living children die, too, when they are a little older, particularly with a seasonal component. Not respiratory, because it's summer time. Not gastro, because it's an agricultural community that isn't taxing its support system. Not the plague; it's the nineteenth century. Not any number of other things (smallpox, etc.) because it's not _one_ particularly dire summer in the area. When I saw malaria as a possibility, I really latched onto it.
The geography of the area in question plays into it. This is all happening in communities like Rockanje, Vierpolders and similar on Voorne-Putten. People who move to Nieuwenhoorn (more salt water and tidal action) or Brielle (drier) don't seem to have the same problem. The problem went away in the 20th century as a result of a variety of changes: screened housing, housing separated from livestock (which was a reservoir), dredging the silted up Bernisse, cleaning out the vegetation from canals. It's also possible that there just isn't anyone really living in the areas most prone to stagnant open water, for that matter.
I wanted to explain this partly for my own benefit, if I should revisit this topic at some point in the future (having by then forgotten most of my thinking) and also because I know I have readers who may well read what I had previously posted and continue to feel very skeptical of my conclusions. If someone has an alternate explanation for the pattern I saw, I'd be interested in hearing that theory. I'm not particularly interested in going over the exact birth dates and so forth in genlias, however, if _you_ want to peruse them, I'd be happy to point you at the names I saw. I didn't even bother to enter the stillbirths in those families in my tree on ancestry.com, because it was just amazingly sad. I did put a note describing what I was seeing in a parent's data.