In oviparous animals the egg retention phase in the uterus is brief. After fertilization eggshell mineralization is triggered but it seems the initial triggers of the pathway is not understood. The uterine fluid is rich in calcium and bicarbonates and calcium carbonate, in the form of calcite, precipitates around the egg. Shell mineralization is quite a rapid process and there are some specific proteins involved in this pathway. Have a look at these:
In ovo-viviparous animals the egg is retained in the uterus for a longer time and embryo is quite developed when the egg is laid (happens in boas). You can call it a sort of "pregnancy".
However progesterone pathway exists in birds and is involved in oocyte maturation
Oxytocin pathways also exist in birds, acting through oxytocin homolog Mesotocin, and perhaps they cause development of maternal care as they do in mammals.
This article talks about effects of oxytocin on birds but not exactly about its role in maternal care.
However i believe that the shell size and mineralization time shouldn't differ much between unfertilized and fertilized eggs in oviparous animals.