The Polish Church Books mention an illegitimate child as "dziecko naturalne". The Polish expression means simply "natural child".
In the index these children are mentioned only with a given name and "dziecko naturalne" added to it.
But there are also cases where a child that has a given name and a last name shows an unobtrusive "d.n." after the name.
In most years there were no "natural" children. But at times there seems to be a significantly increased number of "natural" children. One example can be found in the Church Book of Osiek nad Wisła in the years from 1839 till 1842:
Number of "natural" children
The total number of births doubled from 1838 till 1840 then stayed the same. The number of marriages trebled between 1838 and 1840.
But still so many children without a last name something must have happened during that time period.
The march through of troops or disappointed rebels? Non of the historic events gives a hint in that direction.
The only unusual event reported for that area is that many Polish land owners did not renew the contracts for their German settlers in these years.
But can this explain the increased number of illegitimate births?