Static light scattering measurements performed on poly(N-isopropylacrylamide) hydrogels having a polymer volume fraction around 0.05-0.09 reveal that spatial inhomogeneities grow stronger with rising preparation temperature of the gels, increasing cross-linker concentration, and decreasing monomer concentration. Quantitative analysis of the angular dependence of the excess scattering intensity via the Debye-Bueche method results in correlation lengths of several 10 nm. Relative mean-square concentration fluctuations on this length scale are around 15-35% depending on preparation conditions. The cross-linking efficiency as determined by shear modulus measurements declines when the degree of inhomogeneity is enhanced. Comparison of the experimental results with Panyukov-Rabin (PR) theory indicates that the general trends are well predicted and that thermal concentration fluctuations can be calculated almost quantitatively. However, the theory significantly underestimates the static spatial concentration fluctuations. This discrepancy may be a consequence of preparing the gels by cross-linking polymerization, which leads to much stronger heterogeneity than presumed in PR theory.