The development of fetus in the uterus in mammals is under constant selective pressure. These factors vary among different organisms.
In viviparous species [give birth to the young ones], the mother can buffer the influence of external conditions on the embryos. However, there might be certain pressures that act on the fetus. For instance, when multiple embryos are implanted and developed at the same time, the trait that might facilitate better nutrient acquisition and utilization is favored by natural selection.
On the other hand, in oviparous species [lay eggs] and, the embryos are exposed to different biotic and abiotic factors, such as temperature, moisture, pathogens, or pollutants, which can induce significant variation in the offspring phenotype. Embryos that develop under different thermal conditions might demonstrate different temperature adaptive traits.
Mammalian development in uterus is significantly affected by both biotic [clutch size, presence of parasites in the uterus, and nutrition obtained from the mother] as well as abiotic [heat, cold, pressure, and acidity] factors.